Blood samples were taken from the orbital sinus (venous) of normal (C3H/C(sa)C(sa)), acatalasemic (C3H/C(sb)C(sb)) and hypocatalasemic (acatalasemic heterozygote, C3H/C(sa)C(sb)) mice. Packed red cells prepared from mice were washed three times with cold 0.9% NaCl solution to remove the buffy coat. One volume of red cells was hemolyzed with 1.5 volumes of distilled water. The various mouse hemolysates were fractionated into A, B and C fractions by DEAE cellulose column chromatography with a discontinous buffer system. Catalase activity in the eluate was determined by the perborate method, and the eluate was concentrated to 1.0PU/ml. Agarose isoelectric focusing was performed in a pH 3-to-10 Pharmalyte gradient gel at 81℃. The distribution of catalase in the erythrocyte fractions A,B and C from normal and mutant mice were examined.The activity ratio of the C fraction to the total fraction was greater in the descending order of C(sa)C(sa),C(sa)C(sb) and C(sb)C(sb) mice. Catalase of fraction A of C(sb)C(sb), C(sa)C(sb) and C(sa)C(sa) focused to band at pl 6.5-7.3, pl 5.7-6.5 and pl 5.4-5.9, respectively. Similarly, catalase of the fraction C of C(sb)C(sb), C(sa)C(sb) and C(sa)C(sa) focused to a band at pl 6.3-7.2, pl 5.9-6.5 and pl 5.0-6.0, respectively. Thus the isoelectric points of catalases in fractions A and C obtained from the blood of C(sb)C(sb) were higher than those of C(sa)C(sa). The isoelectric points of catalases in fractions A and C of C(sa)C(sb9 were between those of the C(sb)C(sb) and C(sa)C(sa). The results also indicate that the isoelectric point of catalase in fraction A was higher than that of fraction C in each group of mice.