A method of indirect radioimmunoassay (RIA) for investigation of influenza virus antigenic properties and serum titrations was developed. The method is based on the detection of immobilized specific antigen-antibody complexes using 125I-labeled antispecies IgG. Antigen immobilization is done on the surface of polychlorvynil plate wells at alkaline pH, 37 degrees C for 2 hours. As compared with RIA in solution, the solid-phase RIA is a rapid method, the entire reaction taking 5 hours. The method is highly sensitive (dozen ng), reproducible, and economic. The latter property is important for screening of paired sera during epidemiological surveys of large groups of human population, since the small amount of the blood required may be collected from a finger and not from a vein of the patient. Experiments on competitive inhibition of antigen-antibody reaction done by the solid phase RIA permit quantitative evaluation of antigenic relationships among different influenza virus strains.