Skin fibroblasts from patients with familial polyposis coli (FPC) exhibit increased frequency of morphological transformation, anchorage-independent growth, and enhanced production of viral RNA and viral particles after infection with murine sarcoma virus(murine leukemia virus), MSV(MLV). These FPC fibroblasts form greater amounts of unintegrated viral DNA as compared to normal fibroblasts during the first 4 to 24 hr after infection with Kirsten MSV(KiMLV), suggesting that increased susceptibility of FPC cells to MSV is determined at an early stage of viral transformation. Several MSV-transformed clones of both FPC and normal fibroblasts, isolated from agarose gel, were non-producers having integrated viral genome which was expressed into viral RNA and rescued by infection with MLV.