Radioimmunoassays were developed that can detect antigenic determinants common to mammary tumor viruses (MTV's) of four distinct Mus species: M. musculus, M. cervicolor, M. cookll, and M. caroll. The radioimmunoassays were based on the immunologic cross-reactivity observed between the murine mammary tumor viruses (MuMTV) of M. musculus and type B retrovirus isolated from M. cervicolor. Both of the glycoproteins of MuMTV (gp52, gp36) shared antigenic determinants with virions of M. cervicolor mammary tumor virus. Interspecies radioimmunoassays for gp52 and gp36 were developed and used to detect viruses in the milk of noninbred feral Mus species and MuMTV-related translational products in mammary tumors in these species. Type C and type D retroviruses, as well as the M432 retrovirus of M. cervicolor, did not react in either assay. Both interspecies immunoassays were therefore specific for the detection of distinct MuMTV-related antigenic determinants.