We investigated the production of interleukin (IL)-1 beta and IL-1 receptor antagonist (Ra) by peripheral blood mononuclear cell (PBMC) in vitro during hemodialysis of 7 dialysis patients using 4 differential dialysis membranes (regenerated cellulose [RC], polyamide [PA], polysulfone [PS] and AN-69). Blood sampling was performed before dialysis (0 min), 15 min after starting dialysis, and after dialysis (240 min) during the last session of each treatment. The cellular content of fresh cells and the production of IL-1 beta and IL-1Ra with and without lipopolysaccharide (LPS) stimulation of the cells were evaluated and measured by enzyme-linked immunosorbent assay (ELISA). The level of IL-1 beta with LPS stimulation using RC, PA, and PS membranes was significantly reduced at 15 min and was not changed at 240 min as compared with the level at 0 min. On the other hand, the level of IL-1 beta with LPS stimulation using an AN-69 membrane at 15 and 240 min was not significantly different from that at 0 min. Neither initial cellular content nor spontaneous production of IL-1 beta were detected at 0, 15, or 240 min in any of the membranes. The spontaneous production of IL-1Ra at 15 and 240 min was not significantly different from that at 0 min in any of the membranes. The cellular content of IL-1Ra using the RC membrane was significantly lower at 15 min and did not differ at 240 min from the level at 0 min. The cellular content of IL-1Ra using PA, PS, and AN-69 membranes was not significantly different at 15 and 240 min from that at 0 min. However, the IL-1Ra level with LPS stimulation using RC and PA membranes was significantly reduced from that at 0 min, but the level using PS and AN-69 membranes was not different from that at 0 min. Because IL-1 beta and IL-1Ra levels 15 min after starting dialysis using bioincompatible dialysis membranes were reduced from the levels at 0 min, the findings suggest that measurement of cytokines' during dialysis treatment at an early stage is a useful marker for evaluating the biocompatibility of a dialysis membrane.