Mouse mammary tumor viruses (MMTV) from three different strains of mice have been used to establish productive infections in feline and mink cell lines. The virions that are released by these cells compete completely in a radioimmunoassay for the major virion surface glycoprotein of MMTV (gp52), thus demonstrating that antigenic determinants of gp52 are viral coded. Competitive molecular hybridization studies have shown that the 60 to 70 S RNA's of MMTV's propagated in feline cells contain all the nucleic acid sequences found in 60 to 70 S RNA from MMTV synthesized by murine cells. The virion buoyant densities in sucrose and cesium chloride, virion sedimentation coefficient, divalent cation requirement of the virion DNA polymerase, and morphology of MMTV's synthesized in heterologous cells are similar to those of MMTV's grown in murine cells. Cultures of MMTV-infected feline cells have continuously released between 0.1 and 1.0 microgram of virus per 10(7) cells (75-sq cm flask) per day during the 60-week observation period. No detectable feline or murine type C viruses were produced by these cultures.