The acid and alkaline stability of blood catalase in anemic blood was examined with hemolysates and a crude catalase solution prepared by DEAE column from hemlysates. The results obtained follow. 1. The acid stability of catalase in the hemolysates decreased, in order in acatalasemic, heterozygous hypocatalasemic and normal mice. The acid stability of catalase in the crude enzyme solution from the hemolysates of acatalasemic mice was lower than that of normal or heterozygous hypocatalasemic mice. 2. The acid stability of catalase activity in the hemolysates of blood rich in reticulocytes obtained from anemic acatalasemic mice was higher than that of normal mice. In contrast, the acid stability of catalase in the crude enzyme solution from hemolysate of anemic acatalasemic mice was lower than that of normal mice. 3. Methemoglobin formation in acatalasemic mice's hemolysates in acid solution was more than in normal mice hemolysate. 4. From these results, the acid stability of catalase activity in acatalasemic hemolysates of anemic mice appears to be mainly caused by catalase like activity of methemoglobin. 5. The alkaline stability of catalase at pH 8.0 and 9.0 of both the catalase in the hemolysate and its crude catalase solution from non-anemic mouse blood decreased, in order, from normal, heterozygous hypocatalasemic and acatalasemic mice. 6. The alkaline stability of catalase in the hemolysate and its crude catalase solution from anemic acatalasemic mice were lower than that from normal mice. 7. The acid and alkaline stability of catalase in crude catalase solution from anemic blood of normal and acatalasemia mice was compared to those from non-anemic blood. Catalase from anemic blood was more stable than that from non-anemic blood of acatalasemic mice. A similar tendency to acatalasemia catalase was observed in normal catalase mice.