GRO alpha, a member of the chemokine superfamily, exerts potent stimulatory actions on granulocytes. In this report, we show that activated human polymorphonuclear leukocytes (PMN) are able to produce significant amounts of GRO alpha. Lipopolysaccharide (LPS), tumor necrosis factor-alpha (TNF alpha), and yeast particles opsonized with IgG (Y-IgG) had the ability to induce GRO alpha release, with Y-IgG being the most potent stimulus. The extracellular production of GRO alpha was also modulated by both interferon-gamma (IFN gamma) and interleukin-10 (IL-10). IFN gamma significantly inhibited the production of GRO alpha by PMN stimulated for 2 hr with LPS, TNF alpha, or Y-IgG, but potentiated the production of GRO alpha in cells stimulated for 18 hr with LPS and TNF alpha. IL-10 moderately suppressed the Y-IgG-induced production of GRO alpha, but strongly inhibited the action of LPS and potentiated the effect of TNF alpha. As revealed by Northern blot analysis, the extracellular production of GRO alpha under the experimental conditions used did not always correlate with parallel changes at the level GRO alpha mRNA expression, suggesting that production of GRO alpha by PMN might be regulated at post-transcriptional, translational, or post-translational level. These findings identify a novel biological function of PMN, likely involved in the modulation of the acute inflammatory response.