The production of large macromolecules such as polysaccharides by microbial cells retained behind microporous membranes is subject to mass transfer limitations within the membranes. This paper describes a quantitative investigation of these diffusional limitations in commercially available microporous membrane filters using various dextran fractions as solutes. Dextran diffusivities in membrane pores were determined using a well-stirred diffusion cell. Dextran fractions were characterized by their intrinsic viscosity and weight-average molecular weight (MBAR(w)) determined by light scattering. Hydrodynamic radii (r(H)) were estimated from the macromolecular parameters using the Mark-Houwink and Stokes-Einstein relationships. The tested membranes involved a series of mixed ester cellulose filters with pore radii (r(p)) ranging from 0.11 to 0.6 mum, two track-etched polycarbonate membranes and one inorganic (alumina) membrane. All macromolecules except the smallest (MBAR(w) = 10,500, r(H) = 27 angstrom) showed restricted diffusion in the membranes. Their effective diffusivities (D(p)) were an increasing function of the membrane pore size. For any given membrane, D(p) decreased as MBAR(w) increased. The diffusion data did not fit the Renkin model for a hard sphere diffusing through a cylindrical pore. These results are discussed in terms of membrane and solute characteristics.