Immunoassays are techniques for measuring the concentration or activity of a substance using immunological reactions. Several types of immunoassay are commonly used, including precipitation assays using antibody/antigen complexes, agglutination assays using coated erythrocytes or other particles, and radio-(RIA) and enzyme immunoassays. Multiple factors determine the best assay for a particular use, including sample concentration, technical difficulty, required precision, and availability of specialized equipment. The most sensitive assays are the radio- and enzyme immunoassays, however safety and environmental issues regarding the use of radioisotopes have limited the application of RIAs, and thus enzyme-linked immunosorbent assays (ELISA) tend to be the method of choice when sample concentration is a critical issue. The potential for robotic automation and the availability of inexpensive microplate dilutors, pipetors, and readers has allowed for the generation of highly reproducible results for large numbers of samples in an short time.