Summary. For maximal activation, purified tryptophan oxygenase from rat liver required only half as much preliminary incubation and one-tenth the ascorbate concentration as necessary for the crude enzyme. Purification did not alter the enzyme, but changed its behavior by removing an inhibitory protein. Catalase was identified as the inhibitory protein, addition of which to the purified enzyme reproduced the behavior of crude tryptophan oxygenase. The effect of catalase was localized at the reduction step of the activation. The optimum conditions for the assay of tryptophan oxygenase in different tissue preparations with ascorbate as the reductant are therefore dependent on the catalase content of these preparations in the same way as was previously established with H(2)0(2) as the reductant.