A 67-year-old female diabetic is presented who developed a Coombs'-positive hemolytic anemia after a year of treatment with tolbutamide. An IgG antibody was identified in the patient's serum that caused the agglutination of both the patient's red blood cells and tolbutamide-coated erythrocytes in the absence of complement. Such a reaction did not occur with the patient's erythrocytes when not exposed to tolbutamide. Agglutination of the patient's serum also occurred with erythrocytes treated with other sulfonylureas (chlorpropamide, glibenclamide, carbutamide) but not with phenacetin.