The use of controlled animals, free of microbial contaminants, for animal passages of cells and of controlled raw materials derived from accredited dealers, who are performing viral screens (e.g. for virus absence in serum and trypsin preparations), for the preparation of culture media are steps in the right direction for reducing the risk for viral contaminations. The rules concerning the general cell culture operation procedures for avoiding viral contaminations are largely the same as for preventing mycoplasmal contaminations (see chapter by Drexler and Uphoff).