To determine the effect of virus infection on monocyte chemotaxis, we isolated mononuclear leukocytes (MNL) from human peripheral blood on a Ficoll-Hypaque gradient and quantitated the chemotactic responsiveness in modified Boyden chambers. The MNL were exposed to herpes simplex (7 plaque-forming units/MNL) or influenza virus (103.0 hemagglutinin units/106.7 MNL) before quantification of chemotactic responsiveness of the cells to a chemotactic lymphokine. Incubation of these viruses with human MNL depressed monocyte chemotaxis by 62 to 85% (p <0.001) as compared to MNL incubated with non-infectious (UV-irradiated) viruses or a control tissue culture preparation. Similar amounts of vaccinia, polio, and reovirus were not inhibitory, and suggested that the inhibition was not a universal effect of viruses. Depressed chemotaxis was not caused by virus-induced cytotoxicity (as measured by trypan blue exclusion) since greater than 95% of the MNL were viable at the end of the assay period. The data suggest that one mechanism for the depression of cell-mediated immunity seen in certain viral infections can be due to the ability of viruses to inhibit monocyte chemotactic responsiveness.