Lymphocyte function has been shown to be suppressed during cytomegalovirus infections of humans and mice. We compared nonspecific and specific lymphocyte reactions of splenic lymphocytes from murine cytomegalovirus (MCMV)-infected C3H adult mice during the acute and recovery phases of infection. The response to phytohemagglutinin or lipopolysaccharide was suppressed maximally 4 to 5 days after infection and returned to normal levels by 15 days. The mixed lymphocyte response was suppressed 2 to 10 days after infection and returned to normal by 15 days. In contrast, lymphocytes responded to MCMV-infected syngeneic mouse embryo fibroblasts (MEF) by day 2 of infection incorporating 52,183 ± 9866 counts per minute (cpm) of 3H-thymidine compared with 23,587 ± 3909 cpm with uninfected MEF. The response to infected MEF increased to 132,560 ± 27,739 cpm by day 10. The ability of splenic lymphocytes from infected mice to respond to MCMV-infected MEF was retained for at least 75 days after infection. Preliminary characterization of the cells responding in the MCMV-induced proliferation assay showed that they were sensitive to treatment with anti-θ serum and complement, but not to treatment with anti-mouse γ-globulin and complement. The development of a specific lymphocyte response to viral infected cells during MCMV infection at a time when the MLR and response to mitogens are suppressed suggests that viral induced immuno-suppression may result from the regulation of the immune response of the host rather than a direct effect of the virus.