An antiserum (R2) was raised in a rabbit against dispersed Sprague Dawley rat islet cells. The R2 antiserum contains islet cell surface antibodies, which mediate complement-dependent cytotoxicity against islet cells resulting in a block of glucose induced insulin release. Immunoprecipitation and gel electrophoretic analysis showed that R2 specifically recognizes an Mr 40 000 glycoprotein present in both rat islet and rat insulinoma cells. This glycoprotein is amphiphilic in character and probably represents a pancreatic β cell specific plasma membrane component. The results support previous observations in mouse β cells that a plasma membrane glycoprotein of Mr 40K constitutes a major islet cell immunogen.