Purified prostaglandin H synthase contains cyclooxygenase activity that forms the hydroperoxide, prostaglandin G, and peroxidase activity which removes hydroperoxides. Since hydroperoxides are necessary activators of cyclooxygenase activity, the paradoxical presence of two apparently opposing activities requires careful interpretation. Kinetic studies indicate that the concentration of hydroperoxide needed for full cyclooxygenase activity is much less than that which gives 50 percent effectiveness with the peroxidase. Thus, the peroxidase activity of the synthase is very ineffective in decreasing the hydroperoxide concentration below levels that still permit rapid cyclooxygenase action.