The loss of tryptophan oxygenase from rat livers after induction by hydrocortisone treatment was studied in living rats, isolated perfused livers, liver slices and cell-free preparations of these livers. Measurements of the enzyme by its activity and as an antigen were parallel except for apparently non-specific inactivations in some incubations of cell-free preparations and the temporary, early appearance of a catalytically inactive anitgen during accumulation of the enzyme after hydrocortisone treatment. The enzyme was lost at similar rates in all of the systems studied, including cell-free soluble fraction of liver. In all systems, the antigen was preserved by the substrate analog, α-methyltryptophan. The rate of loss was proportional to the concentration of the enzyme and was the same for added or endogenous enzyme and the same in non-induced or induced liver preparations. Labeled, purified enzyme added to cell-free preparations was used to demonstrate that the loss of antigenic material was not significantly associated with its proteolysis to small fragments. The results are compatible with an initial step in the degradation of this enzyme that is akin to a denaturation, and which can be prevented by combination of the protein with its substrate.