Exposure of various cell types to IFN results in increased enzymatic activities leading to the breakdown of tryptophan and evidence exists to suggest that the catabolism of tryptophan may be connected to the biological effects of interferon (Carlin et al., 1989. The first and rate-limiting enzymatic step in the degradation of tryptophan is the conversion of tryptophan to formylkynurenine. Two different enzymes are deputed to this function. The tryptophan 2,3-dioxygenase (TDO) is found only in the liver and the indoleamine 2,3-dioxygenase (IDO) is found in all tissues examined so far and inducible by IFN and IFN-inducers such as LPS and viruses. Tryptophan catabolism can be demonstrated in vivo in experimental murine models as well as in human subjects (Yoshida et al, 1979). We have confirmed the expression of IDO activity in human monocytes activated by IFN-γ and in the monocytic leukemia cell line THP-1 activated by IFN-γ or by TPA (Musso et al., 1994).