Interleukin-8 (IL-8) has been reported to be released by activated peritoneal macrophages (PMs) and a variety of other cell types, and it exhibits potent chemotactic activity for polymorphonuclear cells (PMNs). We have previously shown that IL-8 is detectable in the drain dialysate of uremic patients on continuous ambulatory peritoneal dialysis (CAPD) during peritonitis. The levels of IL-8 in infected drain dialysate caused by different microorganisms were variable. In this study, we evaluated the gene expression and release of IL-8 by PMs and PMNs during peritonitis caused by Staphylococcus aureus in uremic patients on CAPD. IL-8 levels were variable in the drain dialysate at the different episodes of peritonitis, even in the same patient. The IL-8 levels were highly correlated with PMN count in drain dialysate (r=0.9919, p < 0.001). PMs and PMNs obtained from drain dialysate at the onset of peritonitis increased mRNA expression for IL-8 and the amount of IL-8 mRNA from drainage cells was also highly correlated with PMN count. In contrast, cells isolated from drain dialysate without peritonitis failed to express mRNA for IL-8. These data suggest that increased expression of IL-8 may be a feature of peritonitis. The levels of IL-8 during peritonitis were not only related to the etiological microorganism but also to other unknown factor(s).