Phosphonoformate (PFA) at a concentration of 500 µM and higher completely inhibited the replication of cytomegalovirus (CMV) in cell culture. 50% inhibition was obtained at a concentration of 100 µMPFA. Early nuclear CMV antigens appeared uninhibited, but the formation of nuclear inclusion bodies, late cytoplasmic antigens, Fc-receptors and CPE were all inhibited by PFA. No visible cell toxicity was noted at the PFA concentrations and exposure times used.