Summary The relative amounts of catalase associated with the mitochondrial or cell sap fractions were influenced by the sucrose concentration of the homogenizing medium. The sucrose concentration of the resuspension medium of the mitochondrial fraction was found to affect catalase activity. The different response of mitochondrial and cell sap catalase to deoxycholate, phosphate buffer at pH 2 and 1.25 m sucrose treatment was ascertained. The increase of mitochondrial catalase activity when pretreated with pH 2 phosphate, deoxycholate and 1.25 m sucrose could be explained by the influence of these agents on the permeability of the membrane enveloping this enzyme. Maximum activation of the particulate catalase is concomitant with complete leakage of the enzyme from the particle site. Partial catalase activation is probably due to a combination of leakage and increased permeability of the membrane to H2O2 substrate.