Introduction of two tyrosine residues into human granulocyte colony-stimulating factor (G-CSF) allowed us to obtain radioiodinated material efficiently. By using this material, specific receptors for human G-CSF were analyzed. It was found that human G-CSF receptors were expressed on several myelocytic and monocytic leukemia cell lines. Furthermore, they were also expressed on two choriocarcinoma cell lines. However, the human G-CSF receptors were not detected on other nonhematopoietic tumor cell lines examined, indicating that expression of G-CSF receptors is strictly limited as compared with receptors for granulocyte/macrophage colony-stimulating factor (GM-CSF), which are widely distributed on nonhematopoietic tumors.