A procedure has been developed whereby agar diffusion technics used in bacteriology can be applied to antiviral agents. Chick embryo cell monolayers were grown in Pyrex baking dishes, infected with virus and overlaid with agar. Filter paper discs impregnated with the test materials were deposited on the hardened agar surface. After proper incubation the virus produced plaques in the cell sheet, except in the area around a disc containing an antiviral agent. Since size of the plaque-free zone was proportioned to concentration of the antiviral agent present in the disc, bioassays were simply performed in exactly the same manner as done with antibacterial antibiotics.