Recommendations for fetal bovine serum quality (Hansen and Foster, 1997) . Although the best choice would be a serum-free cell culture process which is devoid of any animal or human derived substances, this is not always possible. Where serum supplementation is necessary, the serum should be of high quality. In addition to the absence of viruses the hemoglobin level should be <10 mg, the endotoxin level should be below 10 Eµ ml −1 , and there should be a reliable tracability to countries without BSE nor foot and mouth disease. A serial filtration using 40 nm poresize filters should be used and the serum should be γ irradiated with >25 kGy using validated procedures. For veterinary use, the radiation dose should be 35 kGy.