A monoclonal antibody has been raised against a surface membrane antigen present on leukemic myeloblasts. In 52 consecutive patients with acute myeloid leukemia treated in a standardized fashion with intensive chemotherapy the immunologic subclass with respect to this antigen was correlated to the clinical outcome. We found the expression of this antigen to predict a poor prognosis, when measured as survival of CR-patients and as survival after 1st relapse.