High titer, monospecific antibodies to human granulocyte myeloperoxidase, cathepsin G, elastase, lysozyme, and lactoferrin were conjugated with fluorescein and rhodamine and used for immunofluorescent staining of mature neutrophils obtained from 25 patients with acute and chronic leukemia. In 11 (44%) of the patients, two populations of mature neutrophils were detected. The abnormal cells were identified by complete deficiency of one or more markers and constituted 10%-100% of the total number of neutrophils. This immunocytochemical approach may permit recognition of mature cells derived from leukemic clones, and serial determinations of the ratio of normal to abnormal cells may be useful in the management of patients with leukemia.