The new biocompatible cellulosic membrane (AM-BC-F [AM-BIO-HX], Asahi-Medical Co. Ltd., Tokyo, Japan) has been developed, which has a higher flux than conventional membranes and more excellent antithrombogenicity because of its smoother membrane surface. The roughness of the inner surface of the AM-BC-F membrane was smaller than that of conventional membranes, as observed by Atomic Force Microscopy, because it was produced by the newly developed spinning method of cuprammonium cellulose solution, which has a different composition from that of a conventional cuprammonium cellulose solution. The degree of platelet adhesion (number of platelets adhered) on the membrane surface was evaluated in vitro by the measurement of the amount of the LDH released from the adhered platelets on the membrane surface after contact with fresh blood of Japanese male white rabbits weighing 2.5-3.0 kg. The number of platelets adhered of AM-BC-F was far smaller than that of conventional membranes. It was deduced from the smoother surface of the membrane. It can be expected that AM-BC-F will have an excellent antithrombogenicity on a dynamic state during actual dialysis treatments, because it is considered that the shearing stress of blood on the inner surface and the interaction between platelets and the membrane surface are less than that of conventional membrane2s.