In order to clarify the interaction of the 11S fraction of soybean proteins with calcium ion, the volume change associated with the interaction was measured with a dilatometer. When CaCl2 was added to 11S protein solution, there appeared a volume increase to a constant saturated value with increasing CaCl2 concentration. Then, the volume decreased at the concentration above 0.04 m CaCl2. On the other hand, the binding of calcium ion to 11S protein was determined by gel filtration at pH 6.0, 7.0 and 8.0. The number of Ca2+ ion bound to 11S protein was evaluated to be 4 to 11 depending on CaCl2 concentration in 0.02 m Tris-HCl and 0.18 m KCl solution at pH 8.0. However, the binding was not observed at pH 6.0 and 7.0 By CD measurements it was shown that the addition of 0.05 m CaCl2 to the protein solution did not cause any change in the secondary structure of the protein molecules.