Nonspecific esterase zymograms of purified leukemic cells from a case of acute myelomonocytic leukemia (AMML) and from a case of acute non-B, non-T lymphocytic leukemia (ALL) were produced by isoelectric focusing and staining with alpha-naphthyl acetate (alpha NA) or alpha-naphthyl butyrate (alpha NB) substrate. A “myeloid” zymogram was found with AMML cells, which closely matched control monocyte and granulocyte zymograms. On the other hand, nonspecific esterase of ALL showed a striking departure from the zymogram pattern of control B lymphocytes and T lymphocytes. An intense reactivity with a very low isoelectric point accounted for most of the ALL nonspecific esterase activity. No corresponding reactivity or relatively small amounts thereof were seen in other zymograms. Conversely, few of the isoenzymes that were prominent in zymograms of control lymphocytes were apparent above trace levels in ALL zymograms. Thus, zymogram analysis of nonspecific esterases clearly differentiated the myeloid leukemia from the lymphoid leukemia and provided a potential marker for each. The AMML cells appeared well enough differentiated with respect to nonspecific esterases as to be similar to mature cells of like lineage. It is plausible that the ALL cells, however, were arrested at an earlier stage of esterase expression as reflected by the associated atypical species.