A radioimmunoassay is a technique typically used to measure concentrations of antigens such as hormones in bodily fluids, without having to use a bioassay.  Radioimmunoassays are highly specific and highly sensitive, as they require the use of radioactive substances.  This technique has now largely been replaced by the Enzyme-linked immunosorbent assay, or ELISA method, which uses colorimetric instead of radioactive signals to detect the presence of antigens.