On August 7, 1975, Kohler and Milstein published in Nature (256:495) a report describing "Continuous cultures of fused cells secreting antibody of predefined specificity. " Their report has become a classic and has already had a profound effect on basic and applied research in biology and medicine. By the time the first Workshop on Lymphocyte Hybridomas (Current Topics in Microbiology and Im munology 81, 1978) was held on April 3-5, 1978, in Bethesda, Maryland, investi gators from many laboratories had made hybrids between plasmacytomas and spleen cells from immunized animals and had obtained monoclonal antibodies reacting with a broad variety of antigenic determinants. At the time Kohler and Milstein introduced this new technology, the editors of this volume were involved in the production of antisera against differentiation antigens (K. B. B. ), histocompatibility antigens (T. ]. McK. ), and human tumor associated antigens (R. H. K. ). Because of the potential usefulness of monoclonal antibodies in these areas, we each began production of hybridomas and analysis of the resulting monoclonal reagents. One of the most interesting aspects of participation in the early stages of the development and application of hybrid oma technology has been observing how the implications of the initial observa tions gradually spread first among the practitioners of immunology and immu nogenetics, and then to other areas of the biological sciences, such as developmental biology, biochemistry, human genetics, and cell and tumor biology.