Membrane marker and cytochemical analyses were carried out on the abnormal cells from 70 adult acute leukemia patients. Such information may (1) supplement standard morphology and serve as a basis for a new classification scheme for acute leukemia, and (2) characterize the surface membranes of granulocyte, lymphocyte, and monocyte “progenitors.” Classification of acute lymphoid leukemias solely on the basis of morphology was unsatisfactory. The presence or absence of T-or B-cell markers was helful in classifying lymphoid leukemias. Monocyte progenitors were characteristically nonspecific esterase positive and Fc-receptor and membrane-IgG positive, but poorly phagocytic. Promyelocytes and myelocytes were frequently Fc-receptor positive and consequently positive for surface immunoglobulin. Myeloblasts were characteristically Fc-receptor negative. We conclude that surface membrane markers are essential in diagnosing lymphoid leukemias and helpful in nonlymphoid acute leukemias, and that cytochemistry is essential in delineating lymphoid from nonlymphoid leukemias and in subclassifying the latter.