Tissue samples from the salivary gland, lung spleen, liver and kidney of Balb/c mice infected with murine cytomegalovirus (MCMV) contained infective MCMV and virus-specific antigens for 6-7 weeks following the infection. After the 8th week no infective virus could be detected in any organ and antigens were found only in the spleen and the mesangial cells of the renal glomerules. As a result of cyclophosphamide treatment applied in the 22nd-24th weeks, the latent viral infection was reactivated in nearly all animals, infective virus production started again in the organs, and lasted for about 3 weeks. During the subsequent latent period the virus was again reactivated by repeated cyclophosphamide treatment. The experimental alternation of the latent and reactivated phases of viral infection renders the model suitable for study of the mechanism and consequences of viral latency and reactivation in vivo.