Acute leukemia was diagnosed in 62 adults and children over a recent 13-month period. Using light microscopy, cytochemical profiles, surface markers, and cytogenetics, 25 cases were classified as acute myeloid leukemia (AML) and 32 as acute lymphoblastic leukemia (ALL). The remaining 5 cases of de novo acute leukemia were unclassifiable. The routine cytochemical battery used on these 62 cases included: myeloperoxidase, sudan black B, nonspecific esterase, and periodic acid-Schiff (PAS). Flow markers utilized were: T3, T4, T5, T8, T10, T11, B1, B4, kappa, lambda, Ia, CALLA, Mo1, Mo2, My4, My7, My8, and My9. TdT was performed by immunoperoxidase and ELISA methods. The five unclassified cases were cytochemically negative and expressed no B- or T-cell-specific antigens, or TdT positivity. The morphologic differential diagnosis was between FAB L-2 and M-1. Karyotypic abnormalities involving chromosomes 3 and 7 were suggestive of myeloid origin in 2 of 4 patients studied. Flow cytometry demonstrated My7 on greater than 50% of blasts from two cases. Myeloperoxidase ultracytochemistry showed reaction product in small primary granules of blasts from all 5 cases. Positive cells contained only 1-2 granules/cell profile. The number of positive cells per case was in the range 10-20%. We conclude from this study that ultracytochemistry is very useful in providing definitive diagnosis and accurate subclassification of some AML FAB M-1 cases, particularly when light microscopic cytochemistry, cytogenetics, and flow cytometric markers are noncontributory. We propose to designate these acute "unclassified" leukemias as AML FAB M-1 "microgranular" type.