Ultrasensitive enzyme immunoassay methods are reviewed not only for antigens but also for antibodies and haptens with emphasis on factors which limit the sensitivity. Ultrasensitive immunoassays can be developed by noncompetitive solid phase assay systems rather than competitive ones for antigens and antibodies. However, no noncompetitive immunoassays have been available for hapten molecules which cannot be bound simultaneously by two different antibody molecules. This has been overcome by developing methods to derivatize haptens with amino groups so that the derivatized haptens may be measured by two-site noncompetitive assays. For ultrasensitive noncompetitive solid phase immunoassays, the nonspecific binding of labeled reactants (background noise) should be minimized. This has been achieved by developing methods to transfer the complex of analytes and labeled reactants from solid phase to solid phase with minimal dissociation of the complex. Thus, the sensitivity for antigens, haptens and antibodies has been markedly improved and some applications have been made.