An autoantibody mimicking anti-S specificity is described in an S-s+ patient. The antibody was detected 2 weeks after transfusion. The direct antiglobulin test (DAT) was positive and the antibody was thought to be an alloantibody formed after the transfusion. Eighteen months later, the DAT was still positive and anti-S could be eluted from the patient's red blood cells in spite of the fact that these were S-s+. The autoantibody was characterized as mimicking anti-S specificity by in vitro absorption studies. It was present during the 3 years in which the patient was observed without causing hemolitic anemia.