Inbred Strain-2 guinea pigs exhibited endogenous peripheral blood mononuclear cell (PBMC)-mediated cytolytic activity against xenogeneic MA104 targets and guinea pig cytomegalovirus (gpCMV)-infected syngeneic and allogeneic targets. This endogenous cytolysis was unaffected by monoclonal T-cell antibody depletion but was diminished by removal of plastic adherent cells. In nonadherent effector populations, cytolysis was mediated predominately by large granular lymphocytes (LGL). During gpCMV infection, cytolysis of both target types was augmented (MA 104 for 3 weeks and gpCMV targets for 10-14 weeks). Augmented cytolysis of gpCMV targets was MHC-unrestricted and was diminished by removal of plastic adherent cells or monoclonal antibody depletion of T-cells, being found largely in LGL enriched populations. A role for this augmented activity in limiting gpCMV infection in inbred guinea pigs is suggested by the temporal association of augmented cytolysis of gpCMV targets with the cessation of viremia and clinical recovery.