To examine binding of recombinant human granulocyte-colony stimulating factor to myeloid cells, the factor was labeled with fluorescein isothiocyanate, and incubated with blood specimens, which were then analyzed by flow cytometry. Neutrophils demonstrated an increased fluorescence, while lymphocytes were negative. These cell fractions were used as controls for cytometric binding assays of leukemic cells. Six patients with lymphocytic leukemia were negative in this assay. Ten of 15 patients with myelocytic leukemia were positive. All patients (n = 5) in myeloblastic crisis of chronic myelogenous leukemia were also positive. The flow cytometry results correlated well with the results of colony formation in response to granulocyte-colony stimulating factor. The results indicate that our method is useful in predicting the susceptibility of leukemic cells to recombinant growth factors.