A semipermeable membrane is described which comprises a homogeneously miscible polymer blend of an aromatic polyamide and polyvinylpyrrolidone. The aromatic polyamide is in particular formed of recurrent units of the formula I: ##STR1## wherein E.sup.1 and E.sup.2 are identical or different and are selected from the groupings ##STR2## --AR.sup.1 --, and --AR.sup.1 --X--AR.sup.2,where AR.sup.1 and Ar.sup.2 are the same or different 1,2-phenylene, 1,3-phenylene or 1,4-phenylene groups which may be substituted by (C.sub.1 -C.sub.6)-alkyl, (C.sub.1 -C.sub.6)-alkoxy, --CF.sub.3 or halogen and X denotesa) a direct bond or one of the following divalent groups: --O--, --C(CF.sub.3).sub.2 --, --SO.sub.2 --, --C(R.sup.1).sub.2 --, with R.sup.1 being hydrogen, (C.sub.1 -C.sub.6)-alkyl or fluoroalkyl having from 1 to 4 carbon atoms in the alkyl group or--Z--AR.sup.1 --Z--, where Z is one of the groups --O-- and --C(CH.sub.3).sub.2 or--O--Ar.sup.1 --Y--Ar.sup.2 --O--, where Y has the meaning given under Xa) above.The membrane has pronouned hydrophilic properties and is moreover stable to hydrolyzing agents and oxidants and to the action of heat. It is also resistant to organic solvents and microorganisms and exhibits low adsorption of proteins. A process for producing the membrane and a process for modifying the retention capacity thereof are also described. The membrane is produced by spreading a solution of the homogeneously miscible polymer blend on a planar substrate and precipitating the blend. Retention capacity is modified by heat treatment.