There are many small black spots on the newly thawed fetal bovine serum under the microscope. Can I apply for a return?

Primary fibroblasts are indeed sensitive to serum quality, but the presence of black spots alone does not mean the serum is unusable. The safest approach is to mix the thawed serum thoroughly, aliquot it into 50 mL tubes, and centrifuge at 400–500 × g for 10 minutes to remove the flocculent precipitate at the bottom. Take the supernatant to prepare complete medium containing 10% FBS, and first test it with one T25 flask of primary cells for 3 days, checking attachment, morphology, and proliferation; if normal, then scale up to subsequent experiments. If the supernatant still contains a large amount of fine particles after centrifugation, it can be further filtered through a 0.45 μm membrane, but note that some macromolecules in the serum may be retained after filtration, and the effect on primary cells varies by cell type. There is also one handling step not to overlook: during thawing, do not leave it at 4°C overnight without movement; gently swirl it every few hours to avoid worsening local precipitation. If it still does not work after such handling, then contact the supplier for a replacement.

After thawing overnight at 4°C, flocculent material at the bottom of the bottle and small black spots of varying sizes are most likely proteins, lipoproteins, and calcium phosphate precipitated at low temperature, especially since imported fetal bovine serum varies between batches. This does not necessarily mean mold or contamination. You can first do two things: first, take a small amount of serum and directly inoculate it into sterile liquid culture medium or a blood agar plate, then culture at 37°C for 48 h to see whether turbidity or microbial growth occurs; second, take centrifuged serum, prepare it according to the normal medium for primary fibroblasts, and perform a small-scale attachment experiment to check the 24 h attachment rate, spreading morphology, and viability. If there is no obvious decrease compared with the old batch, it indicates that it can be used. Do not add the entire bottle directly to primary cells; primary cells are very sensitive to serum batches, so it is best to aliquot it, centrifuge at low speed, and use the supernatant. Warming the precipitate back to 37°C may also partially alleviate it.

Whether a return is possible depends on the evidence. If the supplier says "normal precipitation," that is not necessarily wrong—many serum lots do this after thawing—but when the amount is unusually large, the supplier should also bear responsibility. I suggest you keep the original unaliquoted bottle and the aliquoted tubes, record a video under the microscope, and formally request in writing the certificate of analysis (COA) for that lot, the factory sterility/mycoplasma/endotoxin test data, and the transport temperature records. If these parameters are normal and it is only precipitation, most distributors will exchange the product but may not give a full refund; you can say, "I am concerned about primary cell attachment. If, after centrifugation and inoculation, there is still poor attachment or many dead cells, a new lot will be needed." This is easier to move forward than directly asking for a return. Since reordering takes three weeks, you can first ask them to send a small sample from the same lot or an old lot for emergency use, while continuing to culture with the centrifuged serum so the cells do not run out of medium.