IDO (Indoleamine 2,3-dioxygenase 1) is a heme enzyme that catalyzes the first and rate-limiting step in tryptophan catabolism to N-formyl-kynurenine. This enzyme acts on multiple tryptophan substrates including D-tryptophan, L-tryptophan, 5-hydroxy-tryptophan, tryptamine, and serotonin. Thus, bioactivity of recombinant human IDO was measured through its ability to oxidize L-tryptophan to N-formyl-kynurenine, using Methylene Blue as indicator. Briefly, a mixture of 800μM L-tryptophan, 9000 units/mL catalase, and 40μM Methyene Blue with the addition of 80mM ascorbic acid reaction buffer was incubated with different concentrations of IDO, and the absorbance was read in 321nm every 1.5h. The result indicated that recombinant human IDO oxidize L-tryptophan Enzyme activity of recombinant human IDO.