Hybridoma technology has made possible the production of specific homogeneous antibodies (monoclonal antibodies [MAbs]) with predefined binding characteristics that can be produced in large amounts from immortal cell lines. MAbs can be exquisitely specific, but preparations containing these may be contaminated with tissue-culture additives and nonimmunoglobulin secretion products when produced in vitro, and by host animal proteins when prepared as an ascitic fluid. These contaminants may give rise to artifactual results in immunological methods and will preclude the use of applications that require purified immunoglobulin, such as labeling with radioisotopes, enzymes, and fluorochromes, and coupling to gels for production of immunoaffinity columns. Therefore, some form of purification may be required.