To investigate the relationship between the inflammatory status of T. cruzi-infected Balb/c and B6 mice and the induction of IDO activity and Treg cells, we assessed IDO activity in spleen mononuclear cells (SMC), measured KYN levels in serum, and analyzed the population of Treg cells in the spleen at various time points post-infection (p.i.). The gene discussed is IDO1; the disease is infection.