The passage of human tumor cells in nude mice can also lead to the infection of these cells by murine endogenous retroviruses. Crawford et al. (1979) reported the contamination of a human nasopharyngeal carcinoma with murine endogenous xenotropic retroviruses after a passage in a nude mouse. As these Table V . Viruses potentially infecting rodent cells (Minor, 1996) Group contaminations are of animal origin, it is necessary to verify the contamination status of laboratory animals. Minor (1996) indicated that all mouse strains, which were received at NIBSC from breeders of laboratory animals, were tested positive for MVM and Sendai virus.