The document describes radioimmune assay (RIA), a technique developed in 1959 for measuring hormone levels in plasma. RIA uses radioactive tracers, antibodies, and analytes (the substance being measured) to detect very small quantities as low as trillionths of a gram. It works by having a labeled analyte compete with unlabeled analyte in a sample for binding to antibodies. The amount of labeled analyte bound can then indicate the concentration of unlabeled analyte present. RIA is useful for quantifying hormones, drugs, antigens, and other substances in biological fluids. It provides high sensitivity, specificity, and an indirect method of analysis.