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