Incubation of herpes simplex virus type 1-infected cells with succinyl-concanavalin A, a derivative of the jack bean lectin concanavalin A, resulted in the decreased production of virus. The mode of inhibition by the lectin was unclear. No effect was apparent on the level of viral DNA synthesis. However, incubation of infected cells with increasing concentrations ofthe lectin appeared to result in a decrease in the quantity of viral protein produced within the cell. A reduction in the virus titer of 57 to 64%6 was observed upon direct incubation of extracellular virus in the presence of 50 to 100 pg of succinyl-concanavalin A per ml.