The binding site for calcium(Ⅱ)on concanavalin A(con-A)can be occupied by rareearth ions.When terbium(Ⅲ)ion binds to con-A,its fluorescence intensity is tremendously enhancedbecause of an energy transfer from one or more tryptophan residues in close proximity to the boundterbium(Ⅲ).The dissociation constant of the con-A-Tb(Ⅲ)complex is 8.90×10~(-6)M,obtainedfrom terbium(Ⅲ)fluorescence titration with con-A.The dissociation constant of the complexesof con-A with other rare earth and transition metal ions was obtained through their competition withterbium(Ⅲ)in the reaction with con-A,i.e.the quenching of con-A enhanced fluorescence.Theresults indicate that the binding of rare earth ions with con-A is dependent not only on their ionicradius but also on the hard-soft acid-base strength of the metal ions. The binding site for calcium (II) on concanavalin A (con-A) can be occupied by rareearth ions. When terbium (III) ion binds to con-A, its fluorescence intensity is tremendously enhancedbecause of an energy transfer from one or more tryptophan residues in close proximity to the boundterbium (Ⅲ). The dissociation constant of the con-A-Tb (Ⅲ) complex is 8.90 × 10 -6 M, obtained from terbium (Ⅲ) fluorescence titration with con-A. The dissociation constant of the complexes of con-A with other rare earth and transition metal ions was obtained through their competition with terbium (III) in the reaction with con-A, iethe quenching of con-A enhanced fluorescence. The results indicate that the binding of rare earth ions with con-A is dependent not only on their ionic radius but also on the hard-soft acid-base strength of the metal ions.