Constant transmembrane pressure experiments were made by crossflow filtration to clarify sieving characteristics of microporous glass membranes for plasma fractionation. The distribution of pore diameters is more limited in the microporous glass membranes than in currently utilized synthetic polymer membranes. The filtration resistance of the concentration polarization layer is the dominant factor in plasma fractionation. Proteins are separated more sharply with a higher wall shear rate because of destruction of the concentration polarization layer formed on membrane surfaces. Plasma fractionation using a microporous glass membrane with a pore diameter of 15 nm may allow separation of albumin and IgG at higher wall shear rates. Cascade filtration techniques using microporous glass membranes with various pore diameters may be suitable for plasma fractionation.