An important potential source of viral contaminations are raw materials used for the preparation of culture media in animal cell technology. Although any component of the culture medium can theoretically by contaminated by viruses, the materials with the highest probability of viral contamination are those derived from animal origin. The classical animal cell culture technology currently makes use of several raw materials of animal or human origin. This is true for the production of viral vaccines for human or veterinary use or many other biological therapeutic. Animal sera as medium additive is the most widespread animal derived material used today. Fetal, new born or adult bovine sera and in some cases also horse sera are used. Trypsin, mainly from pig pancreas, is a very important detachment agent for all adherent cells. Other animal or human derived substances are often used in association with the replacement of serum by serumfree media, but which can eventually also be found as excipients: human albumin, protein hydrolysates (casein, gelatin, etc.), and human transferrin. Other substances of animal origin are some amino acids, which are derived from complete hydrolysis of proteins. However, because of the chemical conditions used in their production, there is less risk than that of materials prepared without this process. Although the use of entirely chemically defined media devoid of any animal derived substance reduces the risk of viral contaminations, it is important to mention that this risk will never be zero.