Expression Systems is passionate about baculovirus protein expression technology, and it shows in our commitment to helping our customers obtain the best results they can. Whether you need basic process tips or advanced fine tuning, Expression Systems can help you save time and money by getting the work done right.

We’ve provided answers to our most frequently asked questions below, as well as detailed usage guidelines and informative presentations at right. If you have further technical questions, we welcome you to contact us at or call 530-747-2035.

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In stock items ship same day if ordered by 11AM PST. Items are shipped best way unless requested otherwise by the customer. Perishable products may not be shipped Thursday or Friday depending upon the item and current weather conditions.

Request Fed Ex stickers from Expression Systems and Fed Ex will pick up and return.

You can order directly from this website. Alternately, you can email or FAX us at 530-747-2034.

Warranty: All sales are without any seller’s warranty or representation, expressed or implied, by usage or otherwise; no claims beyond replacement of unacceptable material shall be allowed.


ESF 921, ESF AF and ESF SFM are complete media and include L-Glutamine.

ESF 921, ESF AF and ESF SFM are serum-free media and no addition of serum (FBS) is needed.

ESF 921 and ESF AF consistently out-perform other insect cell culture media products for growth and expression. ESF 921 and ESF AF work with a wider range of insect cells, including Sf9, Sf21, Tni (High Five™) and Drosophila S2.

Our serum-free ESF 921 formulation is much more advanced than these older media products and typically provides much better performance and reproducibility, often at a lower overall cost.  And because it is serum-free and complete, ESF 921 is always ready to use, right out of the bottle.

Our insect media work with nearly all insect expression systems. In addition to our own BestBac, our media is appropriate for Bac-to-Bac®, pFastBac™, MultiBac™, BacPAK,™, BaculoDirect™, and flashBAC.   Add our media to your system for best results.

Expression Systems advises against the addition of antibiotics as they may simply mask low level contaminations which can adversely affect growth and expression.  There are no negative interactions between antibiotics and Expression Systems formulations. If necessary or desired, many antibiotics are suitable for use with insect cells. The following antibiotics are commonly used:

  • Penicillin/Streptomycine: 50-100 U/mL; 50-100 μg/mL
  • Amphotericin B (Fungizone™ antimycotic): 0.25 μg/mL
  • Gentamicin: 0.5 mL of 10 mg/mL solution in 500 mL media (final concentration: 10 μg/mL) Note* When using trypan blue with cultures that have been supplemented with gentamicin, a precipitate can form which can interfere with cell count and viability analysis.

Methionine-deficient media is most commonly used for selenomethionine incorporation for X-ray crystallography.  Methionine is a non-essential amino acid for insect cells and therefore insect cell growth is not impacted by its absence.  Briefly, culture insect cells in ESF 921. When the cells are in log phase of growth, use these cells to seed a culture in 96-200 ESF 921 Delta Series Methionine Deficient Medium at 1 million cells per ml. Grow overnight. The next day, infect the culture with baculovirus containing your gene of interest using an MOI of 3. Culture overnight. The next morning, add selenomethionine at a final concentration of 50-100 mg/L. Culture 24 hours and add selenomethinone as before. Harvest at approximately 72 hours post infection.

No, these chelating agents are not present in ESF 921 or ESF AF.

ESF 921 contains 4.1 grams of NaCl per liter.

Production Boost Additive is a serum-free cocktail of many nutrients and designed for supplementation after infection.  For high MOI cultures, the PBA can be added 6-18 hours after infection and for low MOI infections, PBA can be added 36-48 hours post infection. PBA can be used at a concentration between 1 and 10% of the culture volume, the exact percentage should be determined by titration.

The low pH of insect cell culture media strips the nickel ions from standard chromatography media. When purifying recombinant protein from the culture supernatant, Expression Systems recommends a chromatography resin called Ni Sepharose Excel affinity medium that has the nickel ions very tightly bound. This is available from GE. Insect cell culture media will not strip Ni from this resin, so you can load the clarified supernatant directly onto the column.
Alternatively, a concentration and buffer exchange step may be employed.

We offer 1L bottles, 8L, 10L, 20L and 50L volumes in media transfer bags and an 8L fill in cell culture bags designed to fit rocking bioreactors.

We are media specialists and can produce a custom media to your specifications in volumes from a few dozen liters up to thousands.
Contact or more information.

We understand that our customers can’t afford to wait for shipments or be back-ordered.  So Expression Systems always keeps hundreds of liters of cell culture media in stock at all times.


Insect cells can be frozen in ESF 921 or ESF AF with 10% DMSO. There is no need for the addition of fetal bovine serum.

Expression Systems strongly supports a rigorous observation program of cell cultures rather than relying on a finite number of passages. Culture conditions such as temperature fluctuations, over-growing, under-seeding, oxygen deprivation and other environmental stresses play a far greater role than a passage count. A good rule of thumb is no more than 30 passages as this limits the number of environmental insults the culture has to withstand. Subtle morphological changes in the culture such as cell swelling, elongation or increased clumping are signals to initiate a new culture.

Cell clumping in insect cell cultures is typically a response to environmental stresses. Extended improper passaging techniques, repeated culture overgrowth, seeding at a high density or age of culture can all casue clumping. Solutions are changing the culture flask, following the culture recommendations of the cell line and initiating a new culture from a frozen stock are all solutions.

Chromosome instability in Sf9 cells leads to an accumulation of tetraploid cells. The increased genetic content leads to larger cells. Jarman-Smith,et al Biotechnol Prg 2002 May-Jun; 18(3):623-8. Doverskog et al (Biotechnol Prog. 2000 Sep-Oct;16(5):837-46.) have identified a cellular factor secreted by cells in lag phase that leads to this accumulation of tetraploid cells. To avoid this condition, passage cells while they are still in log phase growth.

Spodoptera frugiperda cells are derived from ovarian tissue of fall armyworm. Sf9 cells are a subclone of Sf21 cells, and were selected for improved growth kinetics. Trichoplusia ni cells are derived from embryonic tissue of the cabbage looper. S2 cells are derived from embryonic tissue of Drosophila melanogaster. We offer our cells in both frozen and suspension formats.

Our suspension insect cells (Sf9, Sf21 and Tni) are a 50 ml culture of insect cells seeded at a density of 1 x 106 cells per ml on the day of shipment. Cells are shipped in 125 ml Erlenmeyer shake flasks. Immediately remove the parafilm and loosen the cap for good aeration. Place the flask in a shaking incubator at 120-140 rpm at 27⁰C. Cells should start doubling 1-2 days after receipt. Refer to enclosed Instructions for Use for detailed culture directions

Accurate titering of recombinant baculovirus is an important step in protein production optimization. But too many labs don’t have the skill or the time to do accurate titering and many even avoid it entirely. Expression Systems offers a very fast and accurate titering service which uses flow cytometric analysis. In just two days, we can give you accurate results. Read more about our titering service here.

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