The influence of blood-membrane interaction on human peripheral blood monocyte tumor necrosis factor-α (TNF), interleukin-6 (IL-6), and interleukin-8 (IL-8) secretion was measured during hemodialysis of end-stage renal disease patients by in vitro stimulation of whole blood with lipopolysaccharide. Monocyte TNF and IL-6 secretion in vitro was reduced 30 min after start of dialysis session. In contrast, cellular IL-8 secretion did not change during hemodialysis. Comparison of the results of three different membranes indicates that the bioincompatibility of the dialysis membrane was reflected in both leukocytopenia and reduction of cellular TNF secretion. During treatment of normal whole blood in an ex vivo dialysis closed-loop circuit, the ability of monocytes to release TNF, IL-6, and IL-8 in vitro remained constant. This indicates that the reduced IL-6 and TNF secretion during standard hemodialysis was not due to a direct effect of contact between dialysis membranes and monocytes, but rather was a result of redistribution within the patients’ leukocyte po