Beta cell and insulin antibodies are involved in the pathogenesis of diabetes in human patients. Beta cell antibodies have also been found in about 50% of newly diagnosed diabetic dogs. This study's objective was to examine these antibodies' role in feline diabetes. The serum of 26 newly diagnosed untreated diabetic cats, 29 cats on insulin therapy, 30 cats with diseases other than diabetes, and 30 healthy cats was examined for beta cell and insulin antibodies. For beta cell antibody testing, purified beta cells from a radiation-induced transplantable rat insulinoma were used. Serum from cats in which anti-beta cell antibodies were induced by injecting a purified beta cell suspension subcutaneously was used as a positive control. Following incubation with test sera, fluorescein-labeled anti-cat immunoglobulins were used to visualize binding between the beta cells and cat gamma globulins. Each serum was tested on two different tumor preparations. For the detection of insulin antibodies, a charcoal separation method was used. It was found that none of the healthy cats, none of the newly diagnosed, untreated diabetic cats and none of the cats with diseases other than diabetes had antibodies against beta cells or against endogenous insulin. Four diabetic cats (14%) that had been treated with different insulin preparations had insulin antibodies. It is concluded that immune-mediated processes are not causing diabetes in the cat. Further studies are needed to evaluate if antibodies directed against exogenous insulin alter the response of diabetic cats to insulin.