1. Radioimmunoassay is an immunoassay technique that uses radiolabeled antigens or antibodies to detect and quantify antigens, antibodies, or other substances in a sample. 2. It works by competitive binding - radiolabeled and unlabeled antigens compete for binding sites on antibodies. The amount of radiolabeled antigen bound is then measured to determine the concentration of unlabeled antigen. 3. RIA is very sensitive and can detect substances at the picogram level. It has applications in detecting hormones, vitamins, drugs, and other molecules.