A rat liver protein with both phosphoenolpyruvate carboxykinase ferroactivator activity and catalase activity has been purified to near-homogeneity. The protein has a native molecular weight of 240,000 and is composed of four identical subunits containing ferriprotoporphyrin IX prosthetic groups. The visible spectrum has absorbance maxima at 403, 500, 530, and 620 nm; it is not reduced by dithionite. The spectrum, physical properties, and specific activity are almost identical with those of catalases from other sources, and the protein has been tentatively identified as rat liver catalase. The protein exhibited partial reactivity in double immunodiffusion plates to antiserum prepared against rat liver ferroactivator isolated by a previous method (Bentle, L. A., and Lardy, H. A. (1977) J. Biol. Chem. 252, 1431-1440) raising the possibility that the original ferroactivator and rat liver catalase are structurally related. Inactivation of catalase by 3-amino-1,2,4-triazole was accompanied by loss of ferroactivator activity as well. The apparent specific activity of ferroactivator, as well. The apparent specific activity of ferroactivator, whether heme-containing or not, can be increased between 2- and 100-fold by the inclusion of bovine serum albumin, HCO3-, or a combination of the two in the incubation.