Human peritoneal mesothelial cells synthesize interleukin-6: Induction by IL-1β and TNFα. Recent studies have demonstrated increased levels of IL-6 in the peritoneal cavity during CAPD peritonitis. The current investigation was initiated (i) to examine the human peritoneal mesothelial cell (HPMC) as a possible source of this secreted IL-6 and (ii) to characterize the released product and examine its regulation by other cytokines. Unstimulated HPMC under growth arrested conditions released IL-6 in a time dependent manner. After 24-hour HPMC IL-6 release (mean ± sem, N = 13) (expressed as pg/μg cell protein) was 1.67 ± 0.33. Stimulation of HPMC with IL-1β or TNFα resulted in a time (increasing up to 48 hr) and dose dependent IL-6 generation. After 24 hours the levels induced by IL-1β and TNFα (both at 1000 pg/ml) were (mean ± sem, N = 13) 19.08 ± 2.98 and 6.62 ± 1.72, respectively. Stimulation with combinations of IL-1β and TNFα resulted in additive increases in IL-6 release. This release could be inhibited by coincubation with anti-IL-1β and/or anti-TNFα antibodies. The level of released HPMC IL-6 measured by immunometric assay (ELISA) correlated directly with that detected in the 7TD1 IL-6 bioassay (r = 0.63; P < 0.001). Western blot analysis of concentrated HPMC supernatants using specific anti-IL-6 antibody demonstrated immunoreactive bands at 23 and 28 Kd following IL-1β or TNFα treatment. PCR amplification of reverse transcribed HPMC mRNA using specific IL-6 primers demonstrated a single 465 base pair transcript, the expression of which was enhanced in a time dependent manner following treatment of HPMC with either IL-1β or TNFα. These data demonstrate for the first time that HPMC synthesize IL-6 and that its release can be regulated as a result of increased expression of specific mRNA and de novo protein synthesis by other cytokines. HPMC derived IL-6 might contribute directly to the cytokine network regulating intraperitoneal inflammatory responses associated with bacterial peritonitis.