The effect of glucocorticoid hormones on the replication of human cytomegalovirus (HCMV) was studied in human embryonic lung (HEL) cells. Treatment of cells with pharmacological concentrations of adrenal glucocorticoids such as dexamethasone enhanced HCMV replication; treatment with oestrogenic or androgenic hormones did not do so. In dexamethasone-treated HEL cells there was an approximately tenfold increase in virus yield, with the virus eclipse period shortened by 1 day compared to control cultures. Treatment of cells with the hormone also enhanced plaquing efficiency of the virus by approximately tenfold. As the synthesis of virus-specific immediate early proteins and antigens was notably enhanced together with an increase of HCMV DNA synthesis, it appeared that the early stages of the HCMV replication cycle might be under hormonal control. Moreover, the data presented suggest that the hormonal enhancement of HCMV replication involves specific receptor proteins and requires the synthesis of a specific cellular mRNA(s)