The existence of the selectively permissive rabbit cell line SIRC allows definition of a new class of endogenous murine type-C virus. Continuous clonal lines of transformed cells derived from the BALB/c mouse-embryo cell line BALB/3T3 contain at least two distinct classes of endogenous type-C viral genomes. Spontaneously released endogenous viruses grow well on the mouse cell line NIH/3T3 (N-tropic viruses) but not on the rabbit cell line SIRC. Type-C viruses induced by treatment with BrdU grow well on SIRC (S-tropic viruses) but not in NIH/3T3 or BALB/3T3. BrdU-treated AKR mouse-embryo cells also release an S-tropic virus. N-tropic and S-tropic viruses both have the mouse intraspecies gs-1 and viral RNA-directed DNA polymerase antigenic determinants. DNA.RNA hybridization techniques reveal that the two host-range classes of endogenous viruses are only partially related to each other. Cell transformation facilitates the spontaneous release of the N-tropic viruses; treatment with thymidine analogues induces the production of the S-tropic viruses. Thus, the two classes of viral genomes appear to be subject to different cellular controls.