A simple antigen-binding assay is described for determining the specificity of monoclonal antibodies against A/Victoria/3/75 influenza virus. Monoclonal antibodies were bound to polyvinylchloride microtitre wells via protein A and anti-immunoglobulin serum. Radiolabelled cell extracts were then added and allowed to adsorb to the antigen. The bound material was eluted and analyzed by polyacrylamide gel electrophoresis and autoradiography. The method can also be used to study competitive binding of antibodies to a given antigen. In this case, one antibody was adsorbed to the wells as above and the competitor antibody was added in high excess with the radioactive antigen. The results obtained with this method are comparable to those obtained by competitive RIA or ELISA.