The assay of red cell superoxide dismutase (SOD) activity employing the method of nitroblue tetrazolium (NBT) reduction in aerobic xanthine oxidase system was studied. The results were as follows. 1, Determination of unit of SOD activity Assuming the range of NBT reduction is 100 percent, one unit is defined by the amount of SOD required to inhibit NBT reduction by 50 percent. However, the activity defined by this calculating method is lower than the real value. Because the maximal inhibition of NBT reduction in aerobic xanthine oxidase system by crude extract was in the range of 75 to 90 percent in this experiment, and differed from each sample. The activities calculated from the point of 50 percent inhibition and half maximal inhibition showed good correlation (correlation coefficient; 0.92). And to get 50 percent inhibition less samples are needed than to get the maximal inhibition. So, this calculating method could be used in the case of comparison of SOD activities. 2, Recovery of purified SOD 93 percent recovery of purified SOD added to the hemolysate before extraction was obtained. The activities were determined 5 times about the same sample to get the variation coefficient. The value was as low as 2 percent. 3, Interferrence by hemoglobin Methemoglobin formation from acatalasemic hemoglobin by SOD reaction was observed. And maximal inhibition of NBT reduction by hemolysate was low and varies widely. Using hemolysate for sample interferes the assay and the value of SOD activity is unreliable.