Pancreatic islet-cell antibodies were demonstrated in the sera of insulin-dependent diabetic patients, with or without polyendocrine disease, by immunofluorescent microscopy in which cryostat sections of fresh-frozen human pancreas were used as the substrate. Sections of Bouin''s-fixed, paraffin-embedded, trypsin-treated human pancreas were used as the islet-cell substrate and the results compared to those obtained with cryostat sections. Sera (24) from insulin-dependent diabetic patients, 1 serum from a non-insulin dependent diabetic patient and 5 sera from normal individuals were tested. With serum samples containing islet-cell antibodies, Bouin''s-fixed, paraffin-embedded tissue produced a more intense fluorescence, was easier to use and was more sensitive than the conventional cryostat method.