The highest risk is associated with the use of human or animal derived substances. With respect to material of human origin, it is evident that there exists an important risk of viral transmission, because of the absence of any species barrier to infection. Human sourced raw materials should be checked for the absence of viruses, like Hepatitis B virus (HBV), Hepatitis C virus (HCV), Human Immunodeficiency virus (HIV), and EBV, or CMV (Committee for Proprietary Medicinal Products: Ad Hoc Working Party on Biotechnology/Pharmacy and Working Party on Safety Medicines, 1992). The testing for these viruses is most frequently accomplished by PCR assay. The extent of the use of human derived materials is limited with only human transferrin and human serum albumin still in use. The development of serum-and protein-free media leads more and more to media devoid of any animal/human derived substance.