The extent of volume fluctuation is calculated for two simple geometrical models of globular protein molecules subjected to a potential that is proportional to the surface area freshly generated by the thermal breathing motion. The proportionality constant, gamma, has the unit of surface tension. The calculated values are compared with estimates made from the compressibility measurements. After an approximate correction for the hydration effect, the experimental values are found to be between those calculated by using gamma values of 25 and 46 cal/mol/A2 (1 cal = 4.184 J). These values bracket previously reported independent estimates of interfacial tension that presumably operates at the interface between a nonpolar molecule and water. This result appears to indicate that the solvent water plays a significant role in determining the extent of volume fluctuation of globular proteins and that the concept, and the actual value of the estimate, of the interfacial tension around a nonpolar molecule in water may, in fact, be useful in some applications.