Many contaminating murine viruses, such as Minute virus of mice (MVM), K virus, Mouse Encephalomyelitis virus, and Mouse Adenovirus have been isolated from contaminated virus pools. Viruses, such as A similar study was published by Nicklas et al. (1993) , however, revealing a lower rate of contamina- (Nicklas et al., 1993) Origin tions probably due to the improvement of the microbiological quality of the laboratory rodents. Of 297 tumors examinated, 75 (25.3%) were contaminated. Considerable differences were observed for in vivo (36.6% positive of 186 tumors) and in vitro (6.3% positive of 111 tumors) passaged transplantable tumors.