The current study focused on the effect of continuous ambulatory peritoneal dialysis (CAPD) dialysate obtained following different intraperitoneal dwell periods on the release of interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNFa) from mononuclear leukocytes (PBMC). Aliquots of 5 X 106/ml healthy peripheral PBMC were exposed to fresh or spent CAPD dialysate (10-240 min of intra-peritoneal dwell) and stimulated with Escherichia coli endotoxin (10 jxg/ml, 2 h). IL-6 and TNFa in cell supernatants were determined by specific enzyme immunoassays. Control PBMC in physiological buffer released 361 ± 70 pg/ml IL-6 and 717 ± 147 pg/ml TNFa (mean ± SEM, n = 8), whereas exposure to fresh dialysis fluids severely suppressed cytokine release from PBMC ( < 30 pg/ml IL-6 and < 15 pg/ml TNFa). A significant inhibition of IL-6 and TNFa release was also observed in PBMC exposed to spent dialysate. The inhibitory capacity of the spent fluids was pronounced with increasing intra-peritoneal dwell time (10 min: 183 ± 45 pg/ml IL-6 and 538 ± 109 pg/ml TNFa; 240 min: 26 ± 5 pg/ml IL-6 and 105 ± 30 pg/ml TNFa; mean ± SEM, n = 16). These data indicate that the impairment of cell responsiveness following exposure of PBMC to peritoneal dialysate is not restricted to the unused fluids, but is also observed following intra-peritoneal equilibration. Moreover, our findings suggest the presence of cytokine inhibitory factors in the peritoneal dialysate of CAPD patients which appear to accumulate in the peritoneal effluent during the CAPD cycle.