Since it is important the availability of a specific marker for interferon induction in vivo, we investigated the effect of different recombinant interferons and various cytokines on indoleamine 2,3-dioxygenase activity. Although with different magnitude, recombinant interferon-alpha A/D (Bgl II) hybrid, interferon-gamma and tumor necrosis factor, all increase the activity of this enzyme, whereas interleukin-1, recombinant interferon-alpha A and interferon-alpha D do not induce this activity in mice lung tissue. Dexamethasone is able to inhibit indoleamine 2,3-dioxygenase induction by lipopolysaccharide or by interferon-alpha A/D but it fails to prevent the induction by interferon-gamma.