The present study uses an in vitro model to test the influence of hormones on the replication and reactivation of cytomegalovirus (CMV) in endometrial tissue culture. Infection of epithelial cells of the endometrium with CMV in the presence of a high concentration of estradiol resulted in only a slightly higher yield of infectious virus. Progesterone did not cause any effect on viral replication. Arrest of CMV replication was achieved by reduction of the pH of the incubation medium. No infectious virus was detectable after incubation of infected cultures at pH 5.8. In the above-described conditions following the arrest of CMV replication by low pH, estradiol treatment was capable of reactivating the virus. The possibility that the increase in CMV infection observed in pregnancy is caused by reactivation of latent virus provoked by hormonal factors will be discussed.