Infection with the minute virus of mice (MVM) is highly prevalent in conventional and SPF mouse breeding colonies; 41 of 52 colonies tested (79%) were infected. Infection is also common in wild mouse populations. Germfree mouse colonies were free of infection. Of 195 serially transplanted mouse tumors and mouse leukemia pools tested, 79 contained MVM as a contaminant. MVM was found in all types of tumors and leukemias without predilection for any particular type. MVM infection in mice is highly contagious, and most mice in breeder colonies become infected during the 2d and 3d months of life. MVM was isolated from the blood and kidneys of naturally infected mice and was in the urine and feces. Serologic tests on sera of human and other animal species, including hamsters, rabbits, guinea pigs, gerbils, and lemmings, revealed no infection. Rat sera frequently contain a hemagglutination inhibition inhibitor, the nature of which, however, is not known.