Copper, zinc superoxide dismutase (Cu2Zn2-SOD) from bovine erythrocyte and its metal ion free derivatives, E2Zn2-SOD, Cu2E2-SOD, and E2E2-SOD (E: empty) were prepared and their secondary structures were investigated by Fourier transform ir spectroscopy. In 20 mM deuterated phosphate buffer (pD 7.5) solution at room temperature, the native Cu2Zn2-SOD contains about 34% beta-strand, 17% beta-turn, and 49% unordered structures, which is similar to the content determined by x-ray crystal structural analysis. The metal ion free derivatives decrease the component of beta-strand and increase the unordered structure component in trend. Especially in the cases of zinc-free derivatives, Cu2E2-SOD and E2E2-SOD, about 24% beta-strand, 20% beta-turn, and 57% unordered structures are obtained. The result indicates that the zinc ion plays an important role in determining the secondary structure of copper,zinc superoxide dismutase.