The heavy-metal chelating properties of a soy protein based hydrogel, prepared by crosslinking an ethylenediaminetetraacetic acid dianhydride (EDTAD) modified soy protein isolate (SPI), have been studied. The equilibrium binding capacities of the divalent calcium, zinc, mercury, and lead ions by the gel were 0.70, 0.65, 0.95, and 0.70 mmol per gram of dry gel, respectively. The distribution ratio of metal ions between the gel and the solution was in the range of 370 to 15,000 mL/g, depending on the initial metal concentration. A positive relationship between the carboxyl group content of EDTAD-modified SPI and the metal-binding ability of the gel was observed; the optimum metal binding occurred at 25ะC. The metal-binding ability increased with increasing pH, in the range where the solubility of the metal ions was not affected by the pH. In binary metal ion solutions, the metal ions adsorbed to the gel in a competitive fashion, influenced by the initial ion concentration. The EDTAD-modified protein hydrogel was readily degraded by proteolytic enzymes and was biodegraded in a fungal overgrowth test. The EDTAD-SPI hydrogel was completely degraded after a 28-day incubation with fungal spores.