The electrophoretic behaviour of rat-liver catalase in polyacrylamide gel depends on the subcellular fraction the enzyme was isolated from. Catalase extracted from the mitochondrial fraction is more anodic than catalase recovered from the unsedimentable fraction of the homogenate. The difference disappears if the sedimentable enzyme is extracted from purified peroxisomes. On the other hand, when treated with a lysosomal extract, catalase present in the unsedimentable fraction or in purified peroxisomes, behaves like the enzyme isolated from the mitochondrial fraction. Factors that influence that effect of lysosomes on catalase indicate that it is due to a proteolysis by an enzyme like cathepsin B. These results suggest that catalase isolated from the mitochondrial fraction differs from catalase isolated from peroxisomes or from the unsedimentable fraction, because it has been subjected to a proteolysis caused by lysosomes present in the mitochondrial fraction, during the extraction procedure. As a matter of fact, catalase isolated from the mitochondrial fraction is endowed with a lower molecular weight than catalase extracted from purified peroxisomes or from the unsedimentable fraction. This structural modification does not apparently affect the stability and the catalytic power of the enzyme.