We have developed a solid-phase radioimmunoassay technique, which allows a large number of samples to be assayed, while minimizing amounts of reagents and technician time. The assay is run in microtiter plates, which results in improved organization, shorter assay set-up time and advantageous miniaturization. On the first day only 3 reagents are required: sample or standard, a 125I-labeled antigen, and an antibody attached to a polyacrylamide bead. After on overnight incubation period, the assay is harvested using a commercial microharvesting apparatus (24 samples/2 min) and placed directly into a gamma counter for counting. In the present study, we have developed the assay to measure secreted IgG, IgM and IgA, and we have compared our results with those of a standard radioimmunoassay technique. This rapid, simple, economical approach should be applicable to the radioimmunoassay of other substances as well.