Congenital neutropenia (Kostmann's syndrome [KS]) is an autosomal recessive syndrome that is characterized by profound neutropenia, resulting in major clinical infections and death. Since the neutropenia and symptoms in KS improve in response to exogenous administration of granulocyte colony-stimulating factor (G-CSF), we studied bone marrow cytokine (G-CSF, granulocyte-macrophage CSF [GM-CSF], and interleukin-6) production under both basal and stimulated conditions. No differences in G-CSF, GM-CSF, or IL-6 gene expression were found in bone marrow stromal cells between normal controls and KS patients, and all three cytokines were detected by enzyme-linked immunosorbent assay (ELISA) in medium conditioned by bone marrow stromal cells from normal donors and patients with KS. Each KS patient tested had detectable, functional G-CSF in their own serum before exogenous G-CSF administration. Since G-CSF production appeared normal in KS patients, we then asked whether we could detect structural defects in the signaling portion of G-CSF receptor genes. Polymerase chain reaction (PCR) amplification of the G-CSF receptor transmembrane region alone, and of the transmembrane plus cytosolic portions of the receptor, yielded the size products predicted from the sequences of the normal G-CSF receptor. Single-strand conformational polymorphism (SSCP) analysis of G-CSF receptor PCR products demonstrated no variance in structural conformation between KS patients and normal subjects. These results demonstrate that bone marrow stromal cells in patients with KS secrete normal concentrations of functional G-CSF and suggest that the neutropenia in KS patients is caused by an inability of neutrophilic progenitor and precursor cells to respond to normal, physiologic levels of G-CSF. Such a defect, clinically responsive to pharmacologic doses of G-CSF, might be caused by defects in the post-G-CSF receptor signal transduction pathway.