Infection of C3H mice with live or UV-inactivated murine cytomegalovirus (MCMV) was able to generate population(s) of lymphocytes in the spleen (CTL) which could exert a lytic effect against L cells infected with MCMV but not against uninfected or those infected with HSV-1. The effector cells proved to be theta-bearing T cells and the lysis of target cells was H-2 restricted. Data presented show that early viral protein synthesis but not viral DNA synthesis was necessary for the appearance of relevant antigenic determinant(s) on target cells. The results of co-capping experiments suggest that H-2 molecules may have close association with MCMV induced product(s) as also with murine leukemia virus glycoprotein (gp70) which is carried by normal L cells. Despite this observation, anti-H-2 serum effectively blocked the cytolysis whereas anti-gp70 and anti-MCMV sera failed. Anti-MCMV serum was effective in blocking cytolysis, only if the L cells were infected for 24 hours and then used as targets. MCMV infected L cells which were coated externally with inactivated Sendai virus could be effectively recognised by MCMV as also by sendai specific CTL. That the cytotoxicity exerted on such targets was of specific nature was revealed by the results of competitive blocking experiments with unlabelled targets.