Binding of Cd2+ to concanavalin A and the subsequent induction of saccharide-binding activity has been studied at pH 6.5. We found that Cd2+ bound to both metal sites, S1 and S2, and that Cd2+ alone would induce sugar binding in concanavalin A. Using the fluorescent sugar 4-methylumbelliferyl alpha-D-mannopyranoside we determined that full saccharide-binding activity was obtained only when the total bound Cd2+ stoichiometry reached 2 ions/concanavalin A subunit. We also report evidence suggesting that the binding of Cd2+ to S2 is the crucial step in activation and that Cd2+ binding to S1 induces a form of concanavalin A similar to that induced by Zn2+, Ni2+, or Co2+ and different from that induced by Mn2+.