A metal combustion promoter is introduced to the circulating inventory of catalyst in a catalytic cracking unit as a mixture of particles rich in metal with particles free of metal such that the net concentration of metal in the mixture is about 1 to 10 ppm. According to a preferred embodiment the particles rich in metal contain about 50 ppm of platinum, iridium, osmium, palladium, rhodium, ruthenium or rhenium. When such mixtures are supplied to the circulating inventory, it is found that specific activity of the metal is enhanced in the sense that activity of the mixture for oxidation of carbon monoxide is higher than that of catalyst in which an equal amount of metal is evenly distributed among all the particles.