Superinduction of IL-6 synthesis in human peritoneal mesothelial cells is related to the induction and stabilization of IL-6 mRNA. The initiation of peritonitis is accompanied by a massive increase in intraperitoneal levels of IL-6. The source of this cytokine and the mechanism by which its levels are increased so dramatically are unknown. We examined the mechanism of IL-6 secretion by HPMC exposed to the milieu present in the peritoneal cavity during the initiation of inflammation. Exposure of HPMC to spent peritoneal dialysis effluent (PDE) or IL-1β resulted in a time- and dose-dependent increase in IL-6 secretion. After 24 hours the IL-6 release (pg/μg cell protein) was increased from 5.0 ± 0.8 in control cells to 16.0 ± 2.4 and to 83.8 ± 17.4 in HPMC treated with PDE and IL-1β (1000 pg/ml), respectively (N = 5, P < 0.05). If, however, PDE and IL-1β were combined, then the secretion of IL-6 was synergistically increased to 747.7 ± 349.9 (P < 0.05 vs. expected additive value). The same effect was evident when PDE was combined with TNFα or mixed with peritoneal macrophage conditioned medium. These changes were not a reflection of HPMC proliferation as estimated by ³H-thymidine incorporation. The “superinduction” of IL-6 release was associated both with the induction and stabilization of IL-6 mRNA. Competitive PCR demonstrated that the amount of IL-6 mRNA (fM/μg total RNA) was increased from 0.35 ± 0.13 in control cells to 11.66 ± 3.89 and to 10.83 ± 5.17 in HPMC treated with PDE and IL-1β (100 pg/ml), respectively (N = 5, P < 0.05). The combination of PDE + IL-1β synergistically increased IL-6 mRNA levels to 56.33 ± 8.79 (P < 0.05 vs. additive value). RNA stability experiments using actinomycin D revealed that the half life of IL-6 mRNA (hours) was increased from 2.8 hours in control cells to 6.7 and 9.4 in HPMC exposed to PDE and IL-1β, respectively. The combination of PDE together with IL-1β resulted in a prolonged stabilization of IL-6 mRNA with levels remaining constant over the 12 hours of the experiment. These data demonstrate that HPMC IL-6 synthesis can be synergistically amplified in the presence of peritoneal dialysis effluent and PMØ-derived cytokines. The results suggest that the peritoneal mesothelium may be responsible for the dramatic rise in IL-6 levels seen during the initial stages of CAPD peritonitis.