Select semipermeable amphiphilic membranes have been prepared and their diffusional characteristics for glucose, insulin, and albumin investigated. The membranes were prepared by cast copolymerization of a hydrophilic monomer (i.e., N,N-dimethyl acrylamide, or N,N-dimethylaminoethyl methacrylate) with the hydrophobic crosslinker methacrylate-ditelechelic polyisobutylene. The products have sufficient mechanical properties for the fabrication of swollen membranes, sheets, tubes, etc. Membranes have been identified which allowed the rapid simultaneous countercurrent diffusion of glucose (Mn = 180 Da) and insulin (Mn = 5733 Da) but did not allow albumin (Mn approximately 60,000 Da) to pass. Evidently, the effective molecular weight cutoff point of these membranes is in the 6-60-KDa range.