A radioimmunoassay was developed to quantitate antitetanus toxoid antibody in whole plasma which can detect 0.75 ng IgG antibody (0.75 micrograms/ml plasma) and 1.60 ng IgM antibody (0.40 micrograms/ml plasma). The IgG antibody was measured by direct binding to a tetanus toxoid-Sepharose immunoadsorbent. The IgM antibody was measured by inhibition of binding by soluble tetanus toxoid because of the relatively high and erratic nonspecific reaction between IgM and the immunoadsorbent. The immunoglobulin standards were [131I]IgG or [131I]IgM coupled to Sepharose, respectively, and the amount of antibody activity in the 125I-labeled antiglobulin reagents could be calculated from the amount bound to the immunoadsorbent and the specific activity of the appropriate immunoglobulin standard. The assay is sensitive, reproducible and suitable for use with large numbers of samples: it should find wide applicability in both clinical and experimental settings.