Mutual diffusion in a multicomponent system at high level of macromolecular crowding has been investigated for a solution containing a protein and a polymeric precipitant. Specifically, precise mutual diffusion coefficients for the ternary system lysozyme ( component 1)-PEG2000 ( component 2)-water ( component 0) up to 25% in volume of PEG have been determined via Gouy interferometry. Experimental diffusion data, D-ij, are compared to those predicted by two semiemprirical equations, which are discussed according to the physics behind them. Dij values are used to analyze the gravitational stability of the protein-polymer aqueous solution, and, in combination with the Onsager reciprocal relations, to evaluate the lysozyme salting out coefficient for addition of PEG (mu(12)). Experimental mu(12) are in good agreement with those predicted by Lekkerkerker's equation. All the results have been compared to those corresponding, previously determined, for a companion system containing a PEG sample at a different molecular weight, PEG400. The influence of PEG-induced crowding on proteins in physiological or in crystallization media has been discussed.