Male mice were inoculated with murine cytomegalovirus (MCMV) to produce an acute generalized infection. Infections virus was recovered from both epididymal sperm and seminal vesicles as well as from uterine sperm collected from mated females, suggesting that MCMV might be transmitted sexually. Because the presence of virus in the ejaculate might affect the fertilization process, the effect of MCMV on the fertilization of mouse gametes and on subsequent embryonic development was studied in vitro. Although the fertilization rate was reduced when sperm were preincubated with infectious virus, this was also the case when heat-inactivated virus was used, leading to the conclusion that this effect was not due to a direct infectious interaction between virus and gametes. Subsequent embryonic development was normal, and there was no evidence of productive infection of the preimplantation embryo.