Diverse alterations in biological properties of CMV-transformed cell lines were observed during prolonged in vitro cultivation. In the CMV-Mj-HEL-2 parent line there was a gradual decrease in the number of cells expressing CMV-related antigens; at the same time, an increase in oncogenicity was observed. One tumour line, designated CMV-Mj-HEL-2,T-1, however, retained the original ratio of cells expressing CMV-related antigens for over 100 in vitro passages. The cells lost their original moderate oncogenicity during this period. A later increase in the ratio of cells without CMV antigenic markers was accompanied by the return of moderate tumorigenicity and karyotypic changes. Both cell lines were studied to determine sensitivity to superinfection with herpesviruses, induction of immune response in nude mice, and release of infectious virus.