Synthesis of cellular protein was substantially inhibited within 1 h of infection with herpes simplex virus, type 2, strain G (HSV-2). The inhibition also occurred, although no virus-specific protein synthesis was detected, after infection with u.v. irradiated virus and in cytoplasts that had been enucleated before infection. The inhibitory activity could not be distinguished from infectivity by dilution, sedimentation or reaction with gamma-globulin. HSV-2 also suppresssed the synthesis of Sendai virus proteins, but not those specified by HSV-1. Host protein synthesis was no more sensitive than virus protein synthesis to an increased concentration of NaCl in the medium, nor could the suppression of host synthesis be prevented by adding excess MgCl2 to the medium or by omitting CaCl2 or NaCl. It was accompanied by the breakdown of polyribosomes, which also occurred in the presence of cycloheximide but not at 4 degrees C. The breakdown yielded ribosomes that were sensitive to a high salt concentration, unlike those produced by treatment of polyribosomes with RNase. The synthesis of cellular DNA and RNA was also inhibited following infection with u.v.-inactivated virus. It is concluded that the suppression of host protein synthesis (and probably also of host DNA and RNA synthesis) is caused by a constituent of the infecting virus particles. The mechanism is obscure but probably does not depend on the leakage out of the cell of Mg2+ or into the cell or Ca2+ or Na+ ions, nor on the specific inhibition of initiation of host polypeptide chains, nor on RNase-like attack on host polyribosomes.