A patient with acute hemolytic anemia and a positive direct antiglobulin test was found to be Jk(a + b +) with anti-Jka in her serum. For 2 weeks prior to admission, the patient had taken chlorpropamide, a hypoglycemic agent. The drug was discontinued upon the diagnosis of hemolytic anemia, and the hemoglobin concentration gradually increased. When chlorpropamide was added to the patient's serum in vitro, it enhanced the reactivity of the anti-Jka, and 40 days posttransfusion, the serum would only react with Jk(a+) red cells when chlorpropamide was present. These findings suggest that a chlorpropamide-dependent antibody with Jka specificity had formed. We do not know why the antibody induced by chlorpropamide reacted preferentially with Jk(a+) red cells.