The Technicon H-1 is a hematology analyzer that performs a complete blood cell count and white blood cell differential using both cytochemistry and flow technology. Two white blood cell cytograms are produced based on peroxidase activity and nuclear characteristics of the cells. Ninety cases of leukemia were studied. The 25 cases of acute lymphocytic leukemia (ALL) could be distinguished from the 39 cases of acute nonlymphocytic leukemia as the lymphocyte percentage was greater than 50% in the ALL cases and the mean peroxidase index value was 0 in 80% of the cases. The ALL cases and the chronic lymphocytic leukemia cases also had different cytogram patterns. Subtypes of acute nonlymphocytic leukemia could not be absolutely distinguished, although promyelocytic leukemias (M3) had characteristic cytograms and a monocyte percentage above 15% suggested a monocytic component (M4 or M5). Chronic myelogenous leukemia likewise seemed to have a recognizable pattern. Since a sample takes only 60 s to process, information is readily available. The unique data available from this instrument should provide a significant advancement in the automated hematology field.