Urogenital tissue specimens were maintained in culture for 2 years. Epithelioid growth was enhanced with use of collagenase digestion rather than trypsinization. Twenty of 34 prostate cancer cell cultures survived more than ten in vitro passages, during which time four of 20 demonstrated epithelioid morphology. One epithelioid line (T-157) survived 32 in vitro passages. The cells demonstrated lack of contact inhibition in culture, were slightly positive in acid phosphatase tests, and reacted positively with cytomegalovirus (CMV)-immune sera in indirect immunofluorescence (IF) tests. These cells, which were proven to be of human male origin, failed to yield infectious virus and could be re-isolated from a nodule induced by the cells when injected sc into weanling athymic nude mice. The serum of the patient from which the tumor cells were derived demonstrated high CMV antibody titers and reacted with the virus-specific membrane and intracellular antigens of CMV-transformed human cells in IF tests. A CMV strain isolated from one of the normal prostate cell cultures established an in vitro long-term persistent infection of human embryo lung cells which resulted in the development of two transformed cell lines. The transformed cells possessed CMV antigenic markers and induced non-differentiated tumors when transplanted into athymic nude mice. The results constitute further evidence of the transforming capacity of CMV, and suggest that the virus may be oncogenic in its natural (human) host.