Human cytomegalovirus (CMV) was inactivated by treatment with phytohemagglutinin (PHA) in contrast to herpes simplex virus (HSV), which was not. Approximately 90% of infectivity was lost following exposure of CMV to PHA. Greater reduction of infectivity, more than 99%, was obtained following pretreatment of cells with PHA than by direct mixture of the virus and the lectin. Protection of cells was still observed 48 hours after pretreatment of cells with PHA. No difference was found in sensitivity to PHA with 3 strains of CMV tested. PHA preparations from different sources were almost equally effective in inactivation of CMV, whereas leucoagglutinin had minimal effect. Our data suggest that the site of action of PHA occurs at the step(s) of penetration and/or uncoating. Resistance of CMV to trypsin was confirmed, and the enzyme had no effect on sensitivity to lectins.