Partial specific volumes at 25 degrees C am reported for alpha-chymotrypsinogen in aqueous solutions containing NaCl, citrate, and/or polyethylene glycol (PEG) over a range of protein concentrations. The concentration dependence of the partial specific volume can be either positive or negative depending on the solvent. For example, the partial specific volume increases with increasing protein concentration in NaCl/citrate solutions at high salt concentrations, and decreases with increasing protein concentration in solutions containing PEG, Kirkwood-Buff solution theory has been applied to interpret these results, and it was found that the concentration dependence is determined by two factors : (1) the effective or solvent-averaged interactions between protein molecules in solution, and (2) three-body protein-protein-solvent and protein-solvent-solvent interactions. An approach is proposed for the experimental determination of both contributions that involves measuring osmotic pressures and volumetric proper-ties of dilute to concentrated protein solutions.