In the ultrafiltration test of a myoglobin solution through porous membranes of the ABA-type block copolymers composed of polyamide as outer segments and polyoxyethylene (M(n) = 1.9-2.0 X 10(4)) as an inner segment, in which the values of weight fraction of the polyamide segments (W) were 0.90, 0.84, 0.82, 0.81, and 0.73, the concentration of the permeate was found to be much higher than that in feed under the pressure difference of 1-2 kg/cm2. Such singularly facilitated permeation was observed also in the test through the dense membranes of the polyamide-polyoxyethylene block copolymer with W of 0.81 and 0.73. On the other hand, neither porous nor dense membranes of the graft copolymer (W = 0.83) having a dextran Stock (M(n) = 1.8 X 10(4)) and 2.6 pieces of polyamide branches showed similar facilitated permeation, although dextran was soluble in water as well as polyoxyethylene. The apparent interaction of myoglobin with dextran may be stronger than that with polyoxyethylene.