An autoantibody was found on the red cells of a patient who had never been transfused previously, but was ill with metastatic adenocarcinoma. The patient's blood group was A1. In addition, the patient's serum and the eluate from the patient's red cells agglutinated A1 but not A2, B, and O red cells. This auto-A1 antibody was reactive at a wide thermal range and was inactivated by dithiothreitol, suggesting the presence of an IgM immunoglobulin. Moreover, the antibody was not associated with a hemolytic anemia.