BALB/c mice were immunized with human islets of Langerhans, and spleen cells from two mice, found to develop cell-surface antibodies against insulin-producing rat islet tumour RIN-5F cells, were fused with mouse myeloma cells. Antibody-producing hybrids were cloned on the basis of their production of surface antibodies reactive with paraformaldehyde-fixed RIN-5F cells by indirect immunofluorescence analysis in the fluorescence-activated cell sorter. Among 236 primary clones, eight stable cell lines producing islet-cell-surface antibodies were eventually cloned. Antibody 2G3 (IgM) reacted with viable normal rat islet cells and high insulin-producing rat islet tumour RIN5-A2 cells, while 3G3 (IgM) only reacted with RIN5-A2 cells. Antibody beta B1 (IgG1) reacted with all islet cells tested and detected an Mr21k component in immunoblotting experiments with RIN-5AH cell plasma membrane proteins electrophoretically transferred to nitrocellulose filters. Antibody 7F6 (IgM) reacted with all islet and non-islet cells tested and detected bands of Mr 66k and 27k by immunoblotting. Antibodies gamma B3, gamma B6, gamma C2, and 6B1 (all IgM) showed varying degrees of binding to different islet cells, but reacted only weakly with non-islet human cells. It is concluded that monoclonal antibodies against pancreatic islet cells may define specific endocrine islet-cell-surface determinants.