Superoxide dismutase (SOD) was isolated from sheep erythrocyte. SOD activity was measured under the optimized assay conditions by observing the variations of autoxidation rate of 6-hydroxydopamine (6-OHDA). The enzyme was characterized as containing copper and zinc and was insensitive to chloroform-ethanol mixture but inhibited by cyanide and hydrogen peroxide. The activity variations and stability properties of sheep erythrocyte Cu, Zn-SOD were investigated under the optimized activity assay conditions by observing inhibition change at the autoxidation rate of 6-OHDA. The optimum pH and temperature of sheep erythrocyte Cu, Zn-SOD were found to be 9.4 and 30 degrees C respectively. The enzyme showed high pH- and thermal-stability properties around neutral pH and up to 37 degrees C after 2.5 h incubation. Variations in the inhibition percentage of autoxidation were investigated in 0.2-0.9 mM range of 6-OHDA. The same procedures were repeated by adding catalase also.