Granulocyte colony-stimulating factor (G-CSF) is a glycoprotein hormone that specifically stimulates both production and functional activation of neutrophils, while interferon-alpha (IFN-alpha) is known to suppress myelopoiesis, including neutrophil production in vivo and in vitro. On a possibility that IFN-alpha may operate as one of the inhibitory feedback factors in neutropoiesis, we examined whether neutrophils produce IFN-alpha in response to G-CSF. Northern blot analysis showed that messenger RNA (mRNA) for human IFN-alpha 1 became detectable time-dependently in highly purified human neutrophils incubated with purified recombinant human G-CSF (rhG-CSF). But such transcription was not observed either in neutrophils incubated with other neutrophil activators, such as formyl-methionyl-leucyl-phenylalanine (fMLP) or lipopolysaccharides (LPS), or in blood mononuclear cells incubated with rhG-CSF. In addition, radioimmunoassay for human IFN-alpha showed that its levels in culture medium of the rhG-CSF-treated neutrophils rose markedly (up to approximately 100 IU/mL/1 x 10(7) cells) in a time-dependent way, compared with those of nonstimulated neutrophils. These findings suggest that the G-CSF/IFN-alpha system may participate in the feedback regulatory loop of neutropoiesis.