Interleukin-8 (IL-8) is a chemoattractant that is highly selective for neutrophils. This study was designed to investigate the presence of IL-8 in peritoneal fluid of patients with acute appendicitis. The clinical circumstances underlying the secretion of IL-8 by mesothelium and its mechanism of activation have not been defined. In an in vitro model for bacterial peritonitis the role of bacteria in activating human mesothelial cells to secrete IL-8 was studied. Cultured human mesothelium was incubated with various species of pathogenic bacteria, isolated from peritoneal exudate fluids of patients with appendicitis. The amount of IL-8 secreted by the cultured mesothelial cells was determined in an IL-8 ELISA, as IL-8 was present in the original peritoneal fluid of these patients. Peritoneal fluids from patients with a perforated appendix were found to contain a significantly higher concentration of IL-8 compared to peritoneal fluids from patients with nonperforating appendicitis (121.6 (57.8) ng/ml versus 0.2 (0.07) ng/ml, respectively; mean (SEM), P < or = 0.01). Species of Bacteroïdes and Fusobacterium necrophorum induced IL-8 secretion from cultured mesothelial monolayers to levels comparable to those found in peritoneal fluids in vivo. Heat-killed bacteria and bacterial supernatant were also able to stimulate mesothelium to secrete IL-8. The results suggest that in the early phase of bacterial peritonitis the influx of PMN is regulated by bacteria-induced IL-8 secretion by the mesothelium lining the peritoneal cavity.