Epstein-Barr virus (EBV) of the transforming and nontransforming strains induced transient stimulation of cellular DNA synthesis during lytic infection of normal mouse lymphocytes. In contrast to human B lymphocytes, the action of both EBV strains in mouse cells was additive. The nontransforming P3HR-1 virus had no cytotoxic effect on mouse lymphocytes. The EBV-infected, stimulated mouse lymphocytes did not express EB virus-determined nuclear antigen and do not grow into immortalized cell lines. The cells expressed EBV-determined early and virus capsid antigens. These results suggest that the stimulation of cellular DNA synthesis by EBV is independent of EBNA synthesis and cell transformation.