Density measurements have been made on aqueous lysozyme solutions at 20, 25, and 30 degrees. The apparent specific volumes, phi v, and expansibilities, phi e, have been determined from the density measurements and fitted to a function of concentration (weight per cent). Sound velocities and heat capacities have also been measured for various concentrations of lysozyme-water solutions at 25 degrees. From the density, expansibility, heat capacity, and sound velocity data at 25 degrees, the isothermal compressibility, phi k, for the lysozyme solutions have been calculated over a range of concentrations. All the physicochemical properties measured were found to be a linear function of the weight per cent of lysozyme. The number of water molecules hydrated to 1 mol of lysozyme was estimated from the volume and compressibility and found to be 162 at 25 degrees.