In order to better understand the patho-physiologic role of granulocyte colony-stimulating factor (G-CSF), we estimated its serum levels in healthy persons and patients with various disorders, using a newly developed enzyme immunoassay (Motojima et al). In 49 of 56 normal healthy persons (88%), the levels were beneath the sensitivity of the assay ( 2,000 pg/mL. Interestingly, a reverse correlation between blood neutrophil count and serum G-CSF level was clearly demonstrated for aplastic anemia (r = -.8169, P < 0.01). Moreover, it was found that the G-CSF level rose during the neutropenic phase of cyclic neutropenia and after chemotherapy or bone marrow transplantation (BMT) in three patients with leukemia; also high G-CSF levels were positively correlated to blood neutrophil counts in some cases of infectious disorders and lung cancer. The cellular sources and the mechanisms for production and secretion of circulating G-CSF were not investigated in this study, but the data presented here strongly indicate that G-CSF plays an important role as a circulating neutrophilopoietin.