The present communication describes the generation of twelve lymphocyte hybrid cell lines whose antibodies react with the rat islet cell line RIN, and the initial characterization of the tissue specificity and functional properties of these antibodies. Since antibodies recognizing islet cell differentiation antigens were sought, these permanent hybrid cell lines were developed from cultures whose antibodies, by radioimmunoassay, bound minimally or not at all to rat red blood cells (cell line A3C1 is an exception). Antibodies from five of the cell lines by radioimmunoassay react significantly with cultured rat fibroblasts in addition to their reaction with RIN cells. Antibody F41-5D6 by indirect immunofluorescence reacted with sections of the RISL transplantable islet cell tumor and specifically with islet cells in sections of rat pancreas. Four antibodies (F41-1G3, 5B5, 6C5, and 5A5), by indirect immunofluorescence, reacted with sections of the RISL transplantable islet cell tumor but not with sections of normal pancreas. Seven of the antibodies were cytotoxic to cultured RIN cells and seven antibodies share the useful property of reacting with protein A. This study demonstrates the feasibility of producing monoclonal antibodies to islet cell differentiation antigens and describes several antibodies which should be useful reagents in studies of the physiology and pathophysiology of the islet cell plasma membrane.