The first step in the kynurenine pathway of tryptophan catabolism is the cleavage of the 2,3-double bond of the indole ring of tryptophan. In mammals, this reaction is performed independently by indoleamine 2,3-dioxygenase-1 (IDO1), tryptophan 2,3-dioxygenase (TDO) and the recently discovered indoleamine 2,3-dioxygenase-2 (IDO2). Here we describe characteristics of a purified recombinant mouse IDO2 enzyme, including its pH stability, thermal stability and structural features. An improved assay system for future studies of recombinant/isolated IDO2 has been developed using cytochrome b (5) as an electron donor. This, the first description of the interaction between IDO2 and cytochrome b (5), provides further evidence of the presence of a physiological electron carrier necessary for activity of enzymes in the "IDO family". Using this assay, the kinetic activity and substrate range of IDO2 were shown to be different to those of IDO1. 1-Methyl-D-tryptophan, a current lead IDO inhibitor used in clinical trials, was a poor inhibitor of both IDO1 and IDO2 activity. This suggests that its immunosuppressive effect may be independent of pharmacological inhibition of IDO enzymes, in the mouse at least. The different biochemical characteristics of the mouse IDO proteins suggest that they have evolved to have distinct biological roles.