Polymethylmethacrylate (PMMA) membrane is the first synthetic polymeric hollow fiber used in dialyzers that is known to adsorb beta(2)-microglobulin. Polyester polymer alloy (PEPA), a blend of two polymers, i.e., polyarylate and polyethersulfone, is another dialysis membrane material with adsorption characteristics. In this study, the adsorption and permeation characteristics of BG-1.6PQ (PMMA) and FLX-15GW (PEPA) dialyzers were investigated by performing ultrafiltration experiments using chymotrypsinogen (molecular weight 25,000) and albumin (molecular weight 66,000) as test solutes. Although PMMA and PEPA had the same sieving coefficient for chymotrypsinogen at steady state, PMMA showed approximately 20% higher fractional adsorption than PEPA under the same initial concentrations. The fractional adsorption for albumin was approximately 20% in PEPA regardless of the ultrafiltration flow rate. The fractional adsorption for albumin in PMMA, however, increased as the ultrafiltration flow rate increased and reached 50%-60% after 10 h. Since PEPA has two skins, one inside and one outside the hollow fiber, proteins may have been adsorbed mainly by these two layers. However, since PMMA is a uniform membrane and since the higher the ultrafiltration flow rate, the higher the fractional adsorption found in PMMA, adsorption may be the result of the occlusion of the dense structure of the membrane. The amount of albumin loss is often clinically evaluated by measuring the amount of permeated albumin in the dialysate. However, when dialyzers with adsorption characteristics are examined, the loss by adsorption should also be taken into account.