Commercially available asymmetric membranes of the Loeb-Sourirajan (L-S) type comprise a support fabric, bonded to the porous substructure. The influence of this fabric on osmotic permeation flux was examined, mostly with a Toray CA-3000 membrane from which, with care, it was possible to remove the support fabric. In osmosis experiments with 12% MgCl2 solution on one side (either side) and 6% solution on the other, the permeation flux (J(1)) was of the order of 0.01 and 0.06 m(3)/m(2) d with and without fabric, respectively. These results could be generalized by considering the resistivity to solute diffusion in the non-skin part of the membrane. This resistivity term averaged 104 and 17 d/m for membranes with and without fabric, respectively, and in further tests without fabric, it was between 15 and 25 d/m over a wide range of MgCl2 concentrations. Four other L-S membranes, all with support fabric, were tested in osmosis experiments. Their resistivity values were similar to or higher than those of the Toray membrane with fabric, but, with one of the four, the results were affected by switching the location of the high and low concentration solutions. It was concluded that existing commercially available L-S membranes are not appropriate for large-scale osmosis applications because their support fabric decreases permeation flux excessively.