Cell separation from peripheral blood was investigated using polyurethane (PU) foaming membranes and PU membranes (pore size, 5 or 12 mum) at different blood permeation speeds. Permeation ratio of hematopoietic stem cells (CD34(+) cells) through the PU membranes was the lowest among the blood cells at any blood permeation speed. This is thought to be because CD34(+) cells are more adhesive than red blood cells (RBCs), platelets, T cells, and B cells. Primitive hematopoietic stem and progenitor cells tend to adhere to the surface of mature blood cells, because of the high expression of cell-adhesion molecules on the surface of the cells. Human serum albumin solution was exposed to PU-COOH membranes to detach adhered cells from the surface of the membranes, allowing isolation of CD34(+) cells and reduction of RBCs in the permeate solution. Most purified CD34(+) cells (high recovery ratio of CD34(+) cells divided by recovery ratio of RBCs) were obtained in the recovery process using PU-COOH membranes (pore size, 5.2 microm) at a permeation speed of 0.3-1 mL/min.