Indoleamine 2,3-dioxygenase (IDO; EC 126.96.36.199) is a rate-limiting enzyme involved in the catabolism of tryptophan, which is an essential amino acid. It is induced under pathological conditions, such as the presence of viral infections or tumour cells. This enzyme is induced by IFN-gamma in the mouse rectal carcinoma cell line CMT-93. It is known that both 1-methyl-L: -tryptophan (1-MT) and methylthiohydantoin-DL: -tryptophan (MTH-trp) are tryptophan analogues, and are authentic inhibitors of the enzymatic activity of IDO. In this study, we examined the effects of both 1-MT and MTH-trp on the IFN-gamma inducible IDO expression of CMT-93. As a result, the IFN-gamma inducible IDO mRNA and the protein levels in CMT-93 were suppressed by 1-MT and MTH-trp, independently. Moreover, tryptophan (Trp), as a substrate of IDO, also suppressed IDO induction by IFN-gamma at the transcriptional level. These results suggest that 1-MT and MTH-trp are as inhibitors of IDO enzymatic activity, and Trp suppresses IDO induction by IFN-gamma at the transcriptional level.