Strains of Marek's disease virus (MDV), a herpesvirus, have been shown to augment the development of lymphoid leukosis induced by retroviruses, the avian leukosis virus (ALV) or the reticuloendotheliosis virus. In this study we explored the possibility that the ability to augment lymphoid leukosis may be correlated with the ability of different strains of MDV to block apoptosis. Subclones of the ALV-transformed B cell line, DT40, which was free of MDV DNA were infected with either R2/23 strain of MDV-1, SB1 strain of MDV-2, or turkey herpes virus (HVT), a MDV-3. We found that most of the normal DT40 cells and DT40 cells infected with the R2/23 became apoptotic when cultured in serum-reduced medium. By contrast, the frequency of apoptotic cells was greatly reduced in the DT40-SB1 and DT40-HVT subclones. These findings suggest that because the SB-1 strain persists in the ALV-infected cells, it may augment lymphoid leukosis by inhibiting apoptosis and providing a survival advantage to the B cells which have a deregulated myc proto-oncogene.