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Using flow cytometry, we quantitatively examined the density of the CD16 (IgG Fc receptor III) antigen on neutrophils in healthy control subjects, in patients with neutrophilia due to bacterial infection, and in patients with chronic myeloproliferative disorders (chronic myeloid leukemia [CML], polycythemia vera, or essential thrombocythemia). The density was expressed as the mean fluorescence intensity of neutrophils stained with fluorescein isothiocyanate–labeled anti-CD16 monoclonal antibody. We also determined leukocyte alkaline phosphatase activity semiquantitatively in the same population. The mean (± SD) density of the CD16 antigen on neutrophils in patients with CML (n = 13; 240.4 ± 134.8) was lower (P<.001) than in healthy control subjects (n = 25; 656.6 ± 238.0), and the density was 288.1), polycythemia vera (n = 7; 552.6 ± 99.9), or essential thrombocythemia (n = 11; 671.5 ± 411.5). The density of the CD16 antigen was 300 or more in all healthy control subjects and in all patients examined, except for those with CML. The CD16 antigen density was less than 300 in 10 of the 13 patients with CML. Leukocyte alkaline phosphatase activity was also low in 10 of the 13 patients with CML. These findings indicate that flow cytometric analysis of the density of neutrophil CD16 antigen is useful for the differential diagnosis of CML from other chronic myeloproliferative disorders.