Abstract 1. 1. The reaction between cytochrome c and cytochrome c peroxidase is first order in cytochrome c. 2. 2. The reaction is inhibited by both oxidized and reduced cytochrome c. 3. 3. The dependence of the reaction rate on ionic strength suggests that the reaction occurs at oppositely charged active sites on the two proteins. 4. 4. The rate of the reaction increases with decreasing pH. 5. 5. The reaction is inhibited by salmine, which suggests that the inhibitory action of cytochrome c is due to its net positive charge in the pH range 5–7. 6. 6. Calculation shows that the rate of reaction may be accounted for on the assumption of an active site of area 5 sq. A. on each of the proteins.