Granule formation was investigated in differentiating neutrophils of a patient with acute myelogenous leukemia (AML) by means of the combined techniques of electron microscopy and peroxidase cytochemistry. Two important pathologic features were observed: first, an abnormal concentration and packaging of peroxidase into Auer rods in leukemic promyelocytes, and second, the presence of Auer rods surrounded by single-unit membranes in some mature polymorphonuclear leukocytes (PMN). An additional unexpected finding was the discovery of two distinct populations of PMN circulating concurrently; a minor (less than 5%) normal one that contained both peroxidase-positive azurophilic and peroxidase-negative specific granules and a major abnormal one characterized by the absence of specific granules. None of these abnormalities was observed during the two remissions of this patient's disease. During relapse a “hiatus leukemicus” occurred, which also revealed two populations of cells, a majority population of leukemic blasts, and a minority population of a few normal PMN. These findings documented several developmental abnormalities in the differentiating cells of myelogenous leukemia and also suggested that concurrent normal and abnormal populations of PMN may be a helpful diagnostic feature of a leukemic process.

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