The effect of a nonadsorbing polysaccharide (dextran) on the structure factor of a solution of lysozyme was studied using small-angle neutron scattering (SANS) experiments. By choosing the appropriate water/deuterium ratio as solvent, we made the scattering signal from dextran invisible for the SANS measurements. Dextran induces a weak long-range attraction between the lysozyme molecules. This attraction is described using a depletion interaction potential from theory for two spheres in an ideal polymer solution. Incorporation of the theory in a mean-spherical approximation shows that the wave vector below which the structure factor increases depends on the polymer size. The theoretical prediction is in fair agreement with the measured structure factor of lysozyme, as affected by nonadsorbing dextran.