The distribution of endogenous type-C RNA viruses was studied in inbred strains of mice and some subspecies of Mus musculus of different geographical origins. The following groups of inducible viruses were characterized by their host range and immunological properties: (1) viruses indistinguishable from one of the three prototype viruses endogenous to BALB/c mice; (2) viruses coding for proteins immunologically related to different prototype endogenous viruses; (3) viruses whose p12 structural proteins were immunologically indistinguishable from that of BALB:virus-2, but whose p30 major structural proteins and envelope glycoproteins differed immunologically from those of previously characterized endogenous viruses. These findings suggest that endogenous viruses have undergone numerous genetic interactions during the process of evolution leading to inducible viruses of present day mouse strains. A class of xenotropic virus spontaneously released by NZB mice is endogenous to but not inducible from embryo cells of other previously studied mouse strains. Viruses which could not be distinguished from the NZB xenotropic virus by host range analysis or radioimmunological techniques were chemically inducible from embryo cells of several mouse strains originating in Asia and Europe. These results indicate that the biological regulatory mechanisms that affect expression of this virus have evolved differently in such strains from control mechanisms that developed in standard inbred strains.