This paper investigates protein absorption and trapping by a membrane in plasma separation from whole blood using polypropylene hollow fibers with 0.5 mum pores. Fresh bovine blood collected with acid citrate dextrose anti-coagulant was circulated through modules with identical membranes but with areas of 0.1, 0.25 and 0.5 m2 while both retentate and permeate were recycled to the reservoir. The amount of adsorbed and trapped proteins was calculated from the protein loss from the reservoir. During steady flow filtration, the protein loss reaches its maximum in 15 to 20 min and is found to be exactly proportional to the membrane area. Even though the sieving coefficient is high (> 0.99) the protein loss reaches approximately 57 g/m2. When pulsations are superimposed on the blood inlet flow in order to increase the permeate flux, the protein loss in instaneously reduced by approximately 50%, probably corresponding to the reversible part of trapping. This is confirmed by SEM photographs of the membranes which show, after steady flow filtration, an almost continuous protein layer while, after pulsations, this layer is seen to have been disrupted. Thus the adsorbed mass may be estimated from the non-reversible loss from the reservoir and represents a layer several molecules thick.