In the presence of granulocyte colony-stimulating factor (G-CSF), the release of IL-1beta and TNF-alpha by LPS-stimulated human whole blood was suppressed. Via measurement of cytokine mRNA, inactive precursor and mature protein, we investigated whether this inhibition occurs at the transcriptional, translational or post-translational level of cytokine production. G-CSF inhibited IL-1beta release, but the formation of proIL-1beta was not attenuated, indicating that G-CSF interferes with the proteolytic processing of proIL-1beta. Since the release of IL-1beta in LPS-stimulated whole blood was blocked by the caspase-1 inhibitor YVAD-cmk, processing of proIL-1beta appears to depend on caspase-1 activity. The conclusion that G-CSF inhibits caspase-1 activity was supported bythe finding that the release of IL-18 was also inhibited by G-CSF, similar to IL-1beta release. Intracellular caspase-1 activity in monocytes was measured by flow cytometry with the cell-permeablecaspase substrate Asp(2)-rhodamine. In the presence of G-CSF the cleavage of this substrate was inhibited by more than 50%. G-CSF had no effect on LPS-induced doubling of caspase-1 mRNA, indicating that G-CSF affects caspase-1 activation and not its formation. For TNF-alpha another mechanism of G-CSF action was identified: TNF-alpha as well as proTNF-alpha formation were inhibited by G-CSF, butG-CSF had no influence on LPS-induced TNF-alpha mRNA level. We therefore suggest that G-CSF causes translational silencing of LPS-induced TNF-alpha mRNA.