Precautions: The precautions which should be taken to reduce the risk of contamination of cells during passage in animals: (i) Cells which have been once passaged in animals have to be screened for the absence of microbial and viral contaminants normally found in the animal species; (ii) Animal passages should be avoided as much as possible, the contamination risk by mouse pathogens is reduced 6-fold when these cells are cultivated in vitro (Nicklas et al., 1993) ; and (iii) only tested laboratory-bred animals (virus-defined, specific pathogen free), and no wild caught animals, should be used for animal passages of mammalian cells.