The virion synthesis by five human cytomegalovirus (CMV) strains in human embryonic fibroblast cultures was stopped by incubation of the infected cultures at 40 degrees C. At this temperature the antigens appeared diffusely filling the nucleus and the cytoplasm. The blocking effect of the elevated temperature was exerted in the same period of the reproduction cycle as the inhibitory effect of cytosine arabinoside (ara-C). In cell cultures infected with CMV and incubated first at 40 degrees C, then at 37 degrees C, the synthesis of infectious virus started again, thus the abortive cycle developed at 40 degrees C was reversible. The inhibition of virus multiplication cannot be attributed to the thermosensitive events in the normal function of the host cell.