Two hemodialysis membranes, polyacrilonitrile (AN 69) and cellulose acetate (CA), were compared for their effects on complement and hemostasis. Two groups of 5 patients, in dialysis for more than 5 years, were successively dialysed for 4 weeks periods with each type of membrane. We measured C3a (complement activation), platelets and beta-thromboglobulin (platelet activation), thrombin-antithrombin III complexes and fibrinopeptide A (coagulation activation), using C-Reactive Protein as a control for dilution effects. As previously shown, activation of complement was more important with CA than with AN 69 (p less than 0.01). In contrast, activation of coagulation (increase in fibrinopeptide A and thrombin-antithrombin III complexes) was more pronounced with AN 69 than with CA. This study emphasizes the need to consider different biological systems when the bioincompatibility of a hemodialysis membrane is evaluated.