Abstract: A hollow fiber membrane ("fiber") comprises a tubular macroporous support coated on its outer surface with a thin tubular asymmetric semipermeable film of polymer. The film, by itself, is non-self-supporting. The support itself is so flexible (flaccid) that it does not have a circular cross-section and collapses with finger pressure. Such a tube of knitted or woven braid no greater than 2.5 mm o.d., having interstitial voids which are non-uniformly shaped by the "ends" which are braided, is coated with a "dope" of polymer, then coagulated in a bath to form the film. Yet, after coating, the braided membrane can withstand up to 1500 psig in hydraulic compression forces before a straight-line relationship between flux and pressure begins to flatten out. The voids are relatively much larger than pores in the film. These voids are small enough to inhibit substantial penetration of the dope which lies superficially supported on the upper portion of the braid.