Guinea pig antiserum containing known quantities of antibody to foot-and-mouth disease virus (140S antigen) and virus protein subunit (12S antigen) was used as a standard in radial immunodiffusion (RID) analyses for determining the antibody content of other antisera. The antibody contents estimated by RID for the unknown sera were in close agreement with those subsequently established by quantitative precipitin analyses. Relatively small amounts of reagents are required and the procedure is simple to perform. Consequently, it provides a feasible procedure for the quantitative estimation of the antibody content of many antisera to the different antigenic components occurring in foot-and-mouth disease. The RID techniques used were approximately 10 times more sensitive for detecting either antibody or virus (140S antigen) than the Ouchterlony double diffusion method. The sensitivity of the procedure used was such that it approached that of complement fixation for the detection of 7S antibody. The greatest sensitivity for detecting either 140S antigen or antibody was obtained when the reactant sought was incorporated into the agar layer. Antibody was detected at a concentration of 1 µg/ml in agar, and 140S antigen was detected at a concentration of 1.25 µg/ml in agar by 19S antibody, and at 2.5 µg/ml by 7S antibody.