To better understand immune mechanisms involved in onset of cytomegalovirus pneumonia, we initially examined the replication of a low virulence strain of mouse cytomegalovirus (MCMV) in nude and BALB/c mice infected by intranasal inoculation. MCMV was detected by plaque assay in the salivary glands of nude mice from days 3 to 16, and in those of BALB/c mice from days 7 to 11. Nude mice became infected with MCMV earlier than BALB/c mice. Moreover, MCMV-DNA was detected in the salivary glands until day 16 after MCMV inoculation in nude and BALB/c mice. However, we did not find evidence of interstitial pneumonia at day 16 in either BALB/c or nude mice. These results suggest that this system represents a latent infection model in BALB/c mice and a persistent infection model in nude mice. We treated latently infected BALB/c mice with methylprednisolone or IL-4 every other day. The mice treated with IL-4 developed interstitial pneumonia, whereas those treated with m-PSL did not. In the present study, we constructed a model of MCMV latent infection that could be used to induce development of interstitial pneumonia. IL-4 appears to be a key cytokine for onset of interstitial pneumonia in mice with latent MCMV infection.