We tried to clarify the membrane permeation mechanism of biopolymer DNA molecule in solution through cuprammonium regenerated cellulose hollow fiber (BMM(TM)) from the dependence of the permeability of the DNA molecule on the molecular weight (MW), the transmembrane pressure (Delta P), the total challenge dose, the original concentration of DNA and on the conformation of DNA and from the observation of the shape of the DNA molecule remaining in the membrane wall. The shape of the DNA molecule was observed using transmission electron microscopy (TEM), The permeability of DNA molecules decreased with an increase in the MW of the DNA molecule. The MW of the molecule which showed a permeability of more than 0.9 was 1 X 10(6) for the protein with global configuration and 1 x 10(8) for DNA, The linear protein of the blood coagulation factor VIII combined with von Willebrand factor (F-VIII with VWF) with MW of 2 X 10(7) showed a permeability similar to that of DNA rather than that of the global protein, When Delta P decreased, the permeability of DNA decreased. Electron microscope observation showed that the DNA molecules were elongated by the shear stress originated in the flow of the solution in pores. We can conclude that : (1) Protein and DNA permeate through BMM mainly based on sieving effects. (2) The molecules of DNA and F-VIII with VWF are considered to deform into a string shape along the stream line. The chemical structure of a molecule and the shear stress of filtration govern its deformability. The deformation of the molecule contributes to the permeability through BMM, (3) The sieving effect in working on the permeation of molecules should take into account their deformability in addition to their geometrical size.