The effects of molecular shape on the osmotic reflection coefficient (sigma(o)) for rigid macromolecules in porous membranes were analyzed using a hydrodynamic model. In this type of model, employed first by Anderson and Malone [1], steric exclusion of the solute from the periphery of the pore induces a concentration-dependent drop in pressure near the pore wall, which in turn causes the osmotic flow. Results were obtained for prolate spheroids (axial ratio, gamma > 1) and oblate spheroids (gamma < 1) in cylindrical and slit pores. Two methods, one of which is novel, were used to compute the transverse pressure variation. Although conceptually different, they yielded very similar results; the merits of each are discussed. For a given value of a/R, where a is the prolate minor semi-axis or oblate major semi-axis and R is the pore radius, sigma(o) increased monotonically with increasing gamma. When expressed as a function of a(SE)/R, where a(SE) is the Stokes-Einstein radius, the effects of molecular shape were less pronounced, but still significant. The trends for slits were qualitatively similar to those for cylindrical pores. When sigma(0) was plotted as a function of the equilibrium partition coefficient, the results for all axial ratios fell on a single curve for a given pore shape, although the curve for cylindrical pores differed from that for slits. For spheres (gamma = 1) in either pore shape, sigma(o) was found to be only slightly smaller than the reflection coefficient for filtration (sigma(f)). That suggests that sigma(o) can be used to estimate at for spheroids, where results are currently lacking. (C) 2007 Elsevier B.V. All rights reserved.