We have isolated an endogenous type C virus from Vandeleuria oleracea (Vand C-I) which by immunological criteria is similar to the type C-I class of endogenous retroviruses previously described in the genus Mus. Interspecies assays for the type B viral associated proteins, gp 52, gp 36, and P28 have been developed. Using these assays, type B viral proteins were found in the milk of M. caroli and M. cookii. A.M. cervicolor popaeus colony was observed to have a high frequency of mammary tumors, type B virus from one of these tumors was transmitted by inoculation to C57B1/6 and Balb/c inbred strains of M. musculus. The major protein (P73) of intracisternal A particles of M. musculus was shown to share antigenic determinants with the major internal protein (p24) of the M. cervicolor endogenous retrovirus M432.