The use of bicarbonate dialysate and high-flux and reprocessed dialyzers has raised concerns about the reverse transfer of dialysate contaminants into the blood compartment. This in vitro study was performed to investigate the reverse transfer of soluble Pseudomonas aeruginosa bacterial products across a polyether sulfone (PES), a newly developed synthetic polymer dialyzer. In vitro dialysis was carried out at 37°C in a closed countercurrent recirculating loop dialysis circuit with a new PES dialyzer. An equal mixture of heparinized whole blood (from healthy volunteers) with pyrogen-free tissue culture medium was circulated in the blood compartment, and bicarbonate dialysate was circulated in the dialysate compartment. After 15 min of dialysis, the dialysate was challenged sequentially with 10<sup>–4</sup>, 10<sup>–3</sup>, and 10<sup>–2</sup> dilutions of a P. aeruginosa culture supernatant. 1-ml samples were drawn from the blood compartment 5 and 15 min after each challenge and incubated upright at 37°C. At the end of 24 h, Triton X-100 was added, in order to extract total interleukin (IL) 6 and IL-8 production by the whole-blood mixture. These cytokines were measured by electrochemiluminescence assays. At dilutions of 10<sup>–4</sup> and 10<sup>–3</sup>, the reverse transfer of soluble bacterial products across the dialyzer was negligible. Five and 15 min after contaminating the dialysate with the highest concentration (10<sup>–2</sup> dilution), the increase in IL-6 production was 239 ± 170% (p = 0.06) and 886 ± 444% (p = 0.02), respectively. However, comparing the IL-6-inducing potency of the 10<sup>–2</sup> bacterial supernatant dilution to the spontaneous IL-6 production in the blood compartment during dialysis with the same dilution of dialysate contaminant, there was a dramatic reduction in IL-6 production by 94 and 89% at 5 and 15 min, respectively. Similarly, 5 and 15 min after contaminating the dialysate with the 10<sup>–2</sup> dilution, the increase in IL-8 production was 357 ± 147% (p = 0.07) and 630 ± 229% (p = 0.04), respectively. However, comparing the IL-8-inducing potency of the 10<sup>–2</sup> bacterial supernatant dilution to the spontaneous IL-8 production in the blood compartment during dialysis with the same dilution of dialysate contaminant, there was a dramatic reduction in IL-8 production by 93 and 92% at 5 and 15 min, respectively. These results demonstrate that PES dialyzers markedly attenuate passage of cytokine-inducing substances from contaminated dialysate, using a method that detects the entire cytokine synthetic output in the blood compartment.