Chronic myelocytic leukemia (CML) is a clonal disorder involving neutrophil, monocyte, erythrocyte, and platelet precursors. In order to determine if the eosinophils are also involved in the leukemic clone, we purified the eosinophils from a woman heterozygous for the common electrophoretic variants of the G6PD gene. Only type B enzyme was demonstrable in the eosinophils, neutrophils, and red cells, but both A and B enzymes were found in the fibroblasts. The data provide evidence that the eosinophil is involved in the malignant clone.