“IMMUNOLOGICAL” techniques (at times pioneered by “endocrinologists”) such as RIAs, immunoradiometric assays, immunocytochemistry, and affinity chromatography are central to current endocrine research. Development of a methodology that would allow the production of unlimited quantities of homogeneous antibody reagents would therefore have a major impact on the field of endocrinology. In addition to the provision of large quantities of homogeneous antibody, monoclonal antibody techniques allow investigators to produce antibodies reacting with single antigenic determinants, despite immunization with impure antigens. The production of monoclonal antibodies to cell surface differentiation antigens after immunization with whole cells is a striking example of immunization with “impure” antigens. In this review we will outline the general methodology and recent developments in the production of monoclonal antibodies and then focus on the many successful applications of these techniques to endocrine research. I. General Methodology The basic techniques for the production of monoclonal antibodies derive from the initial study of Kohler and Milstein in 1975 (1) (Fig. 1).