To examine the antigenic properties of the rat pancreatic beta cell tumor, RIN, we used a cell surface enzyme-linked immunosorbent assay (CELISA). Monoclonal antibodies of known specificity are used to validate the assay, and the results show that antibodies directed at glycoproteins and glycolipids are detected by this technique. The principal glycolipid targets are found in the ganglio and lacto series of glycosphingolipids, and not in the globo series. When the CELISA was used to study sera from type I diabetics, no differences in binding of control and diabetic samples were detected at low dilutions of serum. However, at higher serum dilutions (1/30 to 1/120) binding of IgG antibodies from type I diabetics was greater than that observed with normal sera. Similar reactivity was not detected in these sera when rat hepatocytes were used as targets in the CELISA. Prolonged culture of RIN cells, however, resulted in the loss of reactivity with sera from type I subjects, and is associated with a corresponding decrease in expression of cell surface gangliosides. Accordingly, subclones selected for ganglioside expression were used to compare anti-RIN binding in type I diabetics with that of normal controls and unaffected siblings. The results indicate that some rat islet tumors express antigenic determinants recognized by anti-islet antibodies associated with type I diabetes. Both nonspecific interaction at low serum dilutions and variable expression of cell surface antigens may explain the difficulties encountered when these cells are used for diagnostic purposes. An objective assay such as the CELISA may help to avoid these problems.