BALB/c mice were experimentally infected with murine cytomegalovirus (MCMV) to discover whether latent MCMV persisted in aging mice and to examine the effect of aging on MCMV reactivation. Latently infected mice received saline, cyclophosphamide, or allogeneic blood at 6 and 18 months of age. MCMV DNA was detected by polymerase chain reaction in submaxillary salivary gland biopsy specimens from saline-treated young and old mice. Evidence of MCMV reactivation was sought by culture of biopsy specimens and by MCMV IgG ELISA of pre- and posttreatment sera from all animals. Very few cyclophosphamide- or saline-treated mice reactivated MCMV at either age, but young transfused mice reactivated MCMV significantly more often than did old transfused mice. These experiments indicate that MCMV DNA persists in the salivary gland of aging mice but that the likelihood of MCMV reactivation does not increase with age.