Advances in molecular biology, specifically the rapidly evolving understanding of the molecular basis of disease, generated a need for new assay methods which are quantitative, specific, and ever more sensitive. Nevertheless, the use of radionuclides as immunochemical labels does have certain inherent drawbacks. The relatively short half-life of the gamma-emitting isotope most commonly used in highly sensitive assays limits the useful shelf-life of the reagents. Partly in response to the challenge posed by these apparent drawbacks, a wide variety of nonisotopic immunoassay techniques has arisen. Included in this category are quantitative fluoroimmunoassay, fluorescence polarization immunoassay, free-radical immunoassay, viroimmunoassay, hemeagglutination inhibition, and enzyme-immunoassay. Two of these techniques, such as fluorescence immunoassay and enzyme-immunoassay, have been shown to compete favorably with radioimmunoassay in many areas of performance.