Heat capacities of tetragonal hen egg-white lysozyme crystals containing different amounts of water have been measured In the temperature region between 8 and 300 K, A broad glass transition was observed at about 150 K for the crystals with more than 24.0 wt % water content. These crystals also exhibited a spontaneous exothermic effect due to crystallization of the supercooled water contained in the crystals and melting process of the water above the glass transition temperature. The crystals containing 13.6 and 7.4 wt %, Of Water gave rise to only a glass transition at 165 and 218 K, respectively. The fully dried crystal showed no thermal anomaly in the temperature up to 300 K, The amounts of the freezable and unfreezable water were estimated from the enthalpies of fusion of the crystal water. The water content dependence of the glass transition temperature and the magnitude of the excess heat capacity suggest that the observed glass transition arises from the cooperative motion between the segments of the lysozyme molecules and the bound water molecules.