Examination of the effect of reductants on hematin, which is a prosthetic group of tryptophan oxygenase, showed that glutathione and cysteine changed the optical absorption of hematin and, therefore, the effect of these reductants on tryptophan oxygenase activity was studied. Glutathione and cysteine are both SH-compounds, but their effect on tryptophan oxygenase activity is quite different. Glutathione in 0.1-0.2 mM concentration inhibited tryptophan oxygenase activity, but more than 0.1-0.2 mM of glutathione resulted in recovery of inhibition of tryptophan oxygenase activity, which was activated by 5.0 mM glutathione. On the other hand, cysteine activated tryptophan oxygenase maximally in 0.1-0.2 mM concentration, but more than 0.1-0.2 mM of cysteine rapidly decreased its maximum activity. The inhibition rate of tryptophan oxygenase activity by these reductants was decreased by the addition of purified rat liver catalase.