Passsage of rigid, "spherical" proteins in the range of 2-5 nm radius, R (40-500 kDa molecular weight), through semidilute solutions of a representative polymer, polyethylene glycol in the molecular weight range of (0.6-8) x 10(6), gives rise to a size dependent retardation ("molecular sieving") analogous to that in a porous network composed of random planes. The average distance between those planes being equal to the screening length, xi, the retardation can be described by log(mu/mu(0)) = -(ARc(0.75)) where A is a constant dependent on solvent and type of monomer and c is the polymer concentration.