Ultrafiltration membrane

The invention relates to a hollow-fiber membrane for ultrafiltration made from a hydrophobic aromatic sulfone polymer and at least one hydrophilic polymer, the membrane having an open-pore separating layer on the lumen side, an adjoining supporting layer with asymmetric, sponge-like pore structure without finger pores and an outer layer adjoining the supporting layer towards the outer surface. The separating layer has a cutoff between 20 000 and 200 000 daltons, a thickness of max. 10% of the membrane thickness and an essentially isotropic pore structure. The size of the pores in the supporting layer initially increases up to a zone with maximum pore size, and then decreases towards the outer layer, and the outer layer exhibits a thickness of 10 to 30% of the wall thickness and an essentially isotropic pore structure. The mean size of the pores in the outer layer is larger than in the separating layer, but smaller than in the supporting layer. The wall thickness of the hollow-fiber membrane lies in the range from 100 to 450 μm. The invention relates furthermore to a method for the production of these hollow-fiber membranes.