Blend membranes of cellulose cuoxam with casein were prepared and characterized by SEM, X-ray diffraction, DSC, TG, IR, solid-state NMR, element and amino-acid analysis. The blends were miscible, when the casein content was less than 15% by weight. Young’s modulus, tensile strengths, tearing strengths and breaking elongations were measured, and these values of both the dry and wet blend membranes were superior to that of regenerated cellulose membranes nonblended. Their mean pore diameters and permeabilities were improved. The interaction between the hydroxyl groups of cellulose and peptide bonds of casein in blend membranes is discussed.