Using the polyacrylamide gel disc electrophoresis, the effects of starch on the mobility ofamylases were studied. When the electrophoresis gel contained starch, the mobility of amylases was retarded. The retardation was increased in proportion to starch concentration in the gel. From the change of mobility as a function of starch concentration, the dissociation constants of Taka amylase-A and of EDTA treated human salivary amylase were calculated, to be 2.5×10^-2 g. ml^-1 and 1.9×10^-4 g.ml^-1, respectively. The calculation was based on a theory derived from similar phenomena of chromatography. Therefore, it may also be possible to calculate the dissociation constant of an enzyme from its Rf value on chromatography.