Normal and leukemic hematopoietic cell lysates were labeled with [3H]-diisopropylfluorosorophosphate ([3H]-DFP), an active site inhibitor of serine hydrolases. The labeled proteins in the lysates were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by counting of gel segments for radioactivity. The results indicate the presence of distinct [3H]-DFP binding patterns for different normal and leukemic hematopoietic cells; significantly lower labeling in normal or leukemic lymphoid cells compared to myeloid or monocytoid cells; lower labeling in acute myeloblastic leukemia (FAB-M1) as compared to acute myelomonocytic leukemia (FAB-M4), chronic myelomonocytic leukemia or monocytes and an increase in [3H]-DFP binding with cell maturation along granulocytic series. Thus, these patterns could be useful in discriminating acute lymphoblastic leukemia from myeloid/monocytoid types of leukemia and for following maturation of myeloid cells, and perhaps for studying functional or maturation defects in hematopoietic cells in other pathological conditions.