Membranes with a mean pore diameter of 0.5 mu m were filled with neutral polyacrylamide (PA) gels at fixed crosslink density. The gel was synthesized in the membranes by impregnating the pores with a solution of monomer and crosslinker and then initiating polymerization by ultraviolet radiation. The rate of diffusion and convection (ultrafiltration) of two proteins, ribonuclease A (RNAse) and bovine serum albumin (BSA), was measured as a function of the amount of gel in the pores expressed as the volume fraction (phi) occupied by the polymer forming the gel network. The diffusion rate of potassium chloride (KCl) and glucose was also measured to determine the effects of the gel on small molecules. The effective diffusion coefficient (D-eff), normalized by the diffusion coefficient in bulk solution (D-o), and the reflection coefficient (sigma) were correlated with the parameter (a(s)/a(f))phi where a(s) is the Stokes-Einstein radius of the solute molecule and a(f) is the radius of the PA chains (6 Angstrom) forming the gel network inside the pores. Both D-eff/D-o and 1-sigma decreased sharply as (a(s)/a(f))phi approached the value 0.6. The gel had only a small effect on the diffusion of KCl and glucose. From the experimental results we conclude that gel-filled porous membranes can be fabricated with no significant defects and excellent selectivity based on molecular size.