1. In work along this line it is most important to have a simple method of determining the catalytic activity of tissues. The method must be such that several observations can be made during a single experiment, so that a better idea can be obtained of the velocity of the reaction at various intervals. 2. The catalytic activity of human tissues varies greatly in diseases. A. Nephritis.—The kidney always shows the most marked reduction although the other tissues examined, blood, lung, liver, spleen, likewise show decrease in their power of decomposing hydrogen peroxide. This reduction varies directly with the severity of the pathological lesion in the kidney and the clinical symptoms. The urine also in cases of nephritis shows a much greater inhibiting power than normal urine. This may be accounted for by the reaction of the urine, and subsequent work must prove whether or not the kidney takes any more active part in nephritis and secretes into the blood and urine a substance which manifests itself by a reduction in the catalytic activity. B. The catalytic activity of the blood in the two cases of eclampsia which we have studied was not reduced. This is the most important fact we have so far obtained, if it can be substantiated, since it can furnish us with a ready ante-mortem means of differentiating eclampsia and nephritis. C.Pneumonia.—The lung in the stage of red hepatization has an increased catalytic activity. This increase varies directly with the number of intact red blood cells in the exudate, and in the engorged capillaries. Strength is given to this conclusion by the fact, that on the one hand, there is no increased activity in gray hepatization, while on the other hand, there is an enormously increased activity in the fresh hemorrhagic infarct. D. Tuberculosis.—The decreased activity of the lung in tuberculosis is probably due, for the most part, to the lack of blood in the diseased area, while the lowered activity which is present in the other organs is to be explained by the anæmia and emaciation which accompanies the process. Whether there is a specific catalytic inhibiting substance generated by the process, has not been determined. E. There was no reduction of the catalytic activity in the cases of diabetes mellitus and jaundice studied. F. In the one case of asphyxiation by illuminating gas, there was decided decrease in the catalytic activity of the blood. G. The tissues in the one case of congenital syphilis showed a marked lowering of the catalytic activity. 3. There is a slight decrease in the catalytic activity of the tissues due to post-mortem change, but this is so slight that it is hardly to be taken into consideration in the interpretation of the results obtained. 4. There is no marked change in the catalytic activity due to age. In concluding, we wish to thank Dr. A. S. Loevenhart, at whose suggestion this enzyme was studied, for the unfailing interest with which he has followed the progress of this work, and for the many valuable suggestions he has given us.