Defective murine cytomegalovirus (MCMV) antigen-specific proliferation, induced by treatment of MCMV-infected mice with either antilymphocyte globulin (ALG), prednisolone, or both agents, was eventually restored following the cessation of immunosuppression. At 100 and 278 days following the end of immunosuppressive therapy splenic lymphocytes from infected and subsequently immunosuppressed mice responded significantly in vitro to soluble MCMV antigen after having lost this response immediately upon initiation of immunosuppression. Circulating specific antibodies and mitogen-induced blast transformation were comparable between infected mice and infected mice that also were immunosuppressed. At 278 days following the cessation of immunosuppression splenocytes from infected mice that had been treated with ALG yielded greatly increased background proliferation. Nylon-wool adherence was used to obtain enriched populations of T cells, and B cells and monocytes from MCMV-infected mice. While T cells alone did not respond in vitro to MCMV antigen, recombining B cells and monocytes with the T cells reconstituted in vitro proliferation. Defective lymphocyte proliferation to MCMV for an extended period of time following the end of immunosuppressive therapy indicated a prolonged inability to respond to an active MCMV infection. Identification of the cellular basis for the proliferation defect might lead to the development of effective immunotherapy.