Cytochrome oxidase and peroxidase activities in mammalian red cells are suggested by G- and M-Nadi reactions in rabbit, mouse, and human erythrocytes. Peroxidase activity was present in both the hemoglobin and the nonhemoglobin constituents of the red cells. Removal of oxygen stopped the G-Nadi reaction but not the M-Nadi reaction. Potassium cyanide and sodium azide at a concentration of 10–3-10–4 M partially inhibited G- and M-Nadi reactions. Microscopic examination of reacting cells revealed accumulation of reaction products at the cell periphery. This was not evident when the cells were incubated in a suspension of indophenol blue. Mammalian erythrocytes reduced the tetrazolium salt nitro-BT. Reduction was augmented by removal of oxygen and by cytochrome oxidase inhibitors. Accumulation of formazan at the surface of reacting cells was evident upon microscopic examination. The Nadi reactions were similar whether Ehrlich ascites tumor cells or stroma of hemolyzed erythrocytes were used. Reactions of stroma or whole erythrocytes with nitro-BT were essentially similar. Fixation or heating slowed or abolished all the reactions described.