We studied granulocyte colony-stimulating factor (G-CSF) binding sites on neutrophils from patients with severe congenital neutropenia (SCN; Kostmann-syndrome) and cyclic neutropenia (CN) during treatment with recombinant human (rh) G-CSF. G-CSF receptor expression was measured by scatchard analysis. Neutrophils from six healthy controls expressed between 480 and 1,210 binding sites per cell, whereas neutrophils from five SCN patients expressed increased numbers of G-CSF binding sites ranging between 2,100 and 3,900 per cell. Neutrophils from four patients with CN expressed 350 to 1,600 binding sites per cell. The affinity of rhG-CSF to its receptor was similar in patients and controls. These data suggest that SCN patients and CN patients are not defective in G-CSF receptor expression as judged by the numbers of G- CSF binding sites and binding affinity; however, we cannot exclude defects in parts of the G-CSF receptor that may be involved in the signal transduction pathway.