Purified beta cells from a radiation-induced transplantable rat insulinoma were used to detect beta cell antibodies in serum from untreated diabetic dogs. Serum from dogs in which anti-beta cell antibodies were induced by injecting a purified beta cell suspension subcutaneously was used as positive control. Following incubation with test sera, fluorescein-labeled anti-dog immunoglobulins were used to visualize binding between the beta cells and dog gamma globulins. Nine of the 23 diabetic dogs showed a strongly positive reaction which was characterized by a ring fluorescence, three showed a weak reaction and 11 were negative, i.e. they showed diffuse fluorescence. In contrast, 14 of the 15 healthy dogs showed diffuse fluorescence and one dog showed a weakly positive reaction. Thyroid, liver and kidney cells did not elicit ring fluorescence. Although females (spayed and intact) represented the majority of the diabetic dogs, there was no correlation between sex and the occurrence of antibodies in the diabetic dogs. There was also no correlation to the age of the dogs. In conclusion, we have developed a specific test for anti-beta cell antibodies. The test is reproducible and economical to perform on a large number of samples.