The heat of solidification by freezing of myosin B-0.6M KCl solution and of 0.6M KCl solution alone was determined by use of a water calorimeter. The experimental values thus obtained were 79.3 and 81.2 cal/g for myosin B and 0.6M KCl solutions, respectively. On the other hand, the heat of solidification of 0.6M KCl solution could be calculated from values of its water activity vs. temperature based on Fennema's equation as 75.4cal/g. Both the values for 0.6M KCl solution coincide within the limit of an experimental error of ca. 5cal/g. It was, therefore, assumed that the difference in experimental values for myosin B and 0.6M KCl solution were negligibly small. Furthermore, both solutions solidified at the same temperature. It was concluded that the value of heat of solidification for myosin B-0.6M KCl solution is the same as that for 0.6M KCl solution alone.