The osmotic pressure measurements of bovine immune-gamma globulin in phosphate-buffered solution at pH 7.4 and 0.13 M total salt concentration were extended to near saturation concentrations for ambient temperature. The osmotic pressure at the highest measured concentration of 424 g/L was 4.18 psi (28.3 kPa). A free-solvent model, considering solute-solvent interaction in the concentration variable, provided an excellent fit to observed osmotic pressure nonideality at even the highest protein concentration. The calculated mass of hydrated solvent compared with amounts determined from water-O-17 magnetic resonance for other globular proteins. This model provides an improved correlation to the data over virial equations (truncated to the third term) when only solute-solute interactions are considered. The use of mole fraction as the composition variable was critical in obtaining the excellent fit of the free-solvent model. A combination of the free-solvent correction for the concentration variable coupled with models incorporating solute-solute interaction, such as a virial expansion, will be necessary to generally describe the osmotic pressure of protein solutions for all concentrations.