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Receptor mediated endocytosis of fluorescein isothiocyanate-conjugated heat-aggregated IgG (Fl-HAIgG) via the receptor for IgG (Fc gamma R) was studied using granulocytes from normal donors and patients suffering from chronic myeloid leukemia (CML). Within 15 min of incubation at 37 degrees C, 75% of the normal granulocytes internalized the ligand, while only 13% of the CML granulocytes could internalize the ligand in the same time. This functional defect was seen in all the CML patients analyzed. To elucidate the reason for this defective endocytosis, the Fc gamma R was isolated from normal and leukemic granulocytes and biochemically characterized, by gel electrophoresis, high pressure liquid chromatography, and one- and two-dimensional peptide mapping. Our results show that the molecule from the two cell types is very similar. The defective endocytosis must therefore be due to events which occur after ligand-receptor binding.