In crossflow microfiltration (CMF) of a suspension of Saccharomyces cerevisiae, a dense layer called a "dynamic membrane" was formed with elastic compression at the interface of the membrane surface and the cake layer, and was found to reject bovine serum albumin (BSA) and Dextran T70 (T70). This macromolecule rejection corresponded to the blocking resistance (R(b)), and was dependent on the actual surface pore size (d(m)), the particle size in suspension (d(p)) and the particle compressibility. With S. cerevisiae, when d(m) at 0.5 mum was about one-tenth of d(p) (2-5 mum), the T70 rejection rate increased to 80 % even if d(m) was large compared to the size of T70. The membrane surface was observed with scanning electron microscopy and it was confirmed that a dense layer with compressed cells and biopolymers surrounding the cell was formed on the surface. On the other hand, the T70 rejection was not observed with the cake layer of polymethyl methacrylate particles (d(p) : 0.8 mum) because the particle was incompressible and no dense layer was formed.