Summary Replication of human cytomegalovirus (CMV) was inhibited by 50 µg/ml of rifampin. Nevertheless, a number of functions of CMV were still expressed in the presence of rifampin, including early cell rounding, and the development of immuno-fluorescent antigen, haemadsorption antigen, complement-fixing antigen and precipitin antigens. If rifampin was kept in the culture medium for longer than 48 h, infectious virus was not synthesized, but removal of rifampin resulted in restoration of virus titre within 24 h. In parallel with the restoration of infectivity, removal of the drug resulted in a sharp increase in macromolecular synthesis, first RNA and then virus DNA. The results suggest that rifampin blocks a stage in the production of m-RNA species.