The use of cytochemical procedures in the diagnosis and management of acute and chronic myeloid leukemia. The use of Wright-Giemsa-stained smears alone for the classification of acute leukemias often proves unsatisfactory. Some cases of M1, M5a, M7, and L2 are morphologically similar. In such cases, cytochemical stains can provide an inexpensive and available diagnostic tool. M1 is positive for SBB and MPO. M5a is usually NSE positive, whereas SBB and MPO are negative. M7 usually is ANA esterase, PAS, and AP reactive, and do not stain with SBB, MPO, and ANB esterases. The megakaryocytic lineage usually is confirmed by ultrastructural cytochemistry for PPO or immunocytochemistry for platelet glycoproteins and von Willebrand factor. PAS block positivity and AP dotlike reactivity are suggestive of lymphoid lineage. NSE stains are useful in differentiating M2 from M4. Morphologic and cytochemical techniques also can suggest the presence of certain chromosomal abnormalities such as t(8;21) and inv(16), which may have an influence on prognosis. Because not all cases of acute leukemia are easily subtyped by morphology and cytochemistry, immunophenotyping, karyotyping, and molecular analysis of DNA and RNA of leukemia cells also may be required to define cell lineage. 