Under similar conditions over equal periods of time, Fe++ and Fe+++, hemin and catalase destroyed 10-5 10-2 and 105 moles of H2O2 respectively. Thus, binding of iron into porphyrin increased catalysis by 103, while a decided activation (107) occurred on coupling hemin with cata-lase-protein. Coupling of hemin with simple nitrogenous bases generally led to an increase in catalytic activity, whereby the hemochromogen, with bivalent iron, as catalyzer or catalyzer-yielding substance went to the corresponding parahematin, with trivalent iron. Among the complexes tested, strongest catalytic action was noted for histamines-, imidazole-, and nicotine-parahematins. and 4(5)-methylimidazole-hemochromogen. In no case was the activity in the order of that of catalase. The 4 possible complexes of the photo-hemoglobin series had some activity, which was weakest for hemoglobin and strongest for methemoglobin. The presence of free hema-tin might possibly have interfered. Complexes of globin with other hemins were nearly inactive, with the excep-tion of hemato-methemoglobin which showed high ac-tivity, as did hemato-hemin itself. It is considered improbable therefore that the protein of catalase is identical with globin. Only a little activity was found for a pro-tein-protohemin-complex tested. Helicorubin was found practically inactive, but after addition of pyridine showed high activity. This might be explained on the basis that heliorubin, a compound of protoheme with an unknown nitrogenous carrier, suppresses the catalytic activity of the hemin rest as does globin. Addition of pyridine formed the same pyridine-hemochromogen as from hemoglobin.