During studies on the mechanisms of virus latency, reactivation and resultant tissue injury in mice infected with murine cytomegalovirus (MCMV) in utero or at birth, we found the occurrence of three distinct pathological groups. In the first group, mice died within 4 weeks of exposure to virus and showed evidence of tissue injury due to MCMV in multiple tissues and organs of the body. The second group consisted of mice which survived the initial infection and was composed of a minority (about 25%) which shed virus (chronically infected). The third group (about 75%) consisted of mice in which shedding of virus could not be detected (latently infected). Study of the latter group indicated that virus was not detected in brain, thymus, liver, kidneys, urine or serum by co-cultivation techniques or by cellular DNA-MCMV DNA hybridization. In contrast, virus could be activated from spleen cells by co-cultivation with allogenic but not syngeneic feeder cells and MCMV-DNA was detected in amounts equivalent to 3 to 4 virus genomes per 100 spleen cells. In both the latently infected and chronically infected mice, in all strains studied evidence of virus-antivirus immune complex deposits in the renal glomeruli occurred. Only one of the six infected strains (C57 Br/cdJ) studied showed manifestations of autoimmune disease with the formation of antibodies to nuclear antigens, DNA and soluble nucleoprotein.