The effect in vitro of the naturally occurring flavonoid silybin on human polymorphonuclear leukocyte (PMN) functions has been studied. Preincubation of PMNs for 10 min at 37 degrees C with silybin inhibited, in a dose-dependent way, the luminol-enhanced chemiluminescence (CL) generated by stimulated cells without affecting the non-enhanced CL or superoxide anion production evaluated by the cytochrome C reduction assay. No significant effect of silybin on PMN phagocytic or chemotactic activities were found. Silybin did not absorb light at the wavelength of luminol-enhanced CL and was not toxic to PMNs at the concentrations used. Catalase, a scavenger of H2O2, inhibited luminol-enhanced CL to a similar degree as silybin; moreover, when incubated together with PMNs, silybin and catalase did not produce an additive inhibition of CL. On the contrary, the simultaneous addition of silybin and sodium azide, an inhibitor of myeloperoxidase, further increased inhibition over that seen with azide alone. These results suggest that inhibition of H2O2 may be the mechanism by which silybin inhibits the luminol-enhanced CL generated by stimulated PMNs. Such results indicate a possible anti-inflammatory activity for silybin even if their clinical relevance remains to be elucidated.