For insight into the thermodynamics and phase behavior in concentrated protein solutions, we study the liquid-liquid phase separation with lysozyme. We determine independently the binodal and spinodal lines, and the second virial coefficient of the protein at 275 < T < 295 K. From these data, we determine the protein chemical potential and osmotic pressure for concentrations as high as 320 mg mL(-1) in the above temperature range. We find that for this protein, the enthalpy of the liquid-liquid separation vanishes at the critical temperature, T-c, and is comparable to and may exceed the crystallization enthalpy (similar to65 kJ mol(-1)) at lower Ts. The enthalpy of the pair interactions averaged over all polar angles is significantly lower; this comparison suggests structuring of the dense liquid. We propose that the pair of parameters (molecular volume, second virial coefficient) may be an adequate predictor of the phase behavior of solutions of proteins with relatively simple interaction potentials.