We have already reported a high rate of occurrence of antimicrosomal antibodies in diabetes mellitus. Thirteen of 507 diabetics (2.5%) were positive with antithyroglobulin antibodies and thirty-one (6.1%) were positive with antimicrosomal antibodies compared to 2.3% and 2.5% respectively in normal controls. Two of 34 insulin dependent diabetics (5.9%) were positive with antithyroglobulin antibodies and ten (29.4%) were positive with antimicrosomal antibodies compared to 2.3% and 4.4% respectively in 473 insulin independent diabetics. To clarify the association of insulin antibodies and thyroid antibodies in diabetics, antithyroid antibodies in 507 diabetics were tested by tanned red cell hemagglutination test and insulin antibodies were demonstrated by using a method descrived by Wright in a modified form. Twelve of 482 diabetics negative-insulin antibody (2.5%) were positive with antithyroglobulin antibodies and thirty(6.2%)were positive with antimicrosomal antibodies. Only one of 25 positive-insulin antibody (4%) was positive with antithyroid antibodies respectively. No evident correlation was observed between antithyroid antibodies and insulin antibodies.