Bifunctional antibodies are monovalent, bispecific, antibody-derived molecules. They have been produced by both chemical and biological means. They are thought to have several advantages over monoclonal antibodies in both immunotherapy and immunodiagnosis. Bifunctional antibodies have been shown to be efficient in the targeting of drugs, toxins, radiolabelled haptens and effector cells on to diseased tissues, primarily cancer cells. In addition, bifunctional antibodies have been used to develop novel immunoassays. The full potential of bifunctional antibodies has yet to be realised.