Monoclonal antibodies Leu 11a (CD16) and Leu 19 (CD56) were tested for reactivity with cells from 36 patients with acute myelogenous leukemia (AML) using two-colour flow cytometry. Blast cells were identified by a broad panel of monoclonal antibodies. In 33% (12/36) the monoclonal antibody Leu 19, which has been demonstrated to bind to the 140 kD isoform of the human neural cellular adhesion molecule N-CAM, found on peripheral natural killer (NK)-cells, neuroectodermal cells, activated T-cells, and myeloma cells, was shown to bind strongly to the leukemic cells. The monoclonal antibody Leu 11a, which recognizes a surface differentiation antigen associated with the low affinity FcRIII for IgG, expressed on NK-cells, granulocytes and macrophages were found to bind to leukemic cells of four of the 12 Leu-19 positive cases. 50% (6/12) of Leu-19 positive patients were classified as having M4 according to the French-American-British (FAB) morphology criteria. The potential diagnostic and clinical importance of CD 56 and/or CD 16 expression in acute myelogenous leukemia is presently under investigation.