We studied the in vitro effects of granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin 5 (IL-5) on nuclear segmentation of neutrophils and eosinophils from three patients with congenital or acquired Pelger-Huët anomaly. After a 24-h incubation with G-CSF, the majority of neutrophils showed nuclear development characterized by a bilobed appearance. In contrast to neutrophils, the nuclear segmentation of eosinophils was not induced after incubation with G-CSF, GM-CSF, or IL-5. These results suggest that G-CSF plays some role in the nuclear development of neutrophils, whereas IL-5 may not have such an effect on eosinophil maturation in the individual cases studied.