The interferon (IFN)-γ-induced indoleamine 2, 3-dioxygenase (IDO) is implicated in the inhibition of intracellular pathogens, e.g. Chlamydia psittaci and Toxoplasma gondii. The intracellular signaling molecules responsible for the induction of IDO gene expression were investigated by the quantitative polymerase chain reaction. The gene expression was inhibited by a tyrosine kinase inhibitor, genistein. Being consistent with this, IFN-γ induced increased tyrosine phosphorylation and this was inhibited by genistein. The transcription of IDO gene was not inhibited by protein kinase C (PKC) inhibitors, H-7 and staurosporin, or a calmodulin inhibitor, W-7. Irrelevance of PKC in IDO gene expression was supported by the failure of PMA or PMA+A23187 to induce IDO gene expression. These results all suggest that the tyrosine phosphorylation is a critical event in IFN-γ-inducible IDO gene expression and PKC is not involved in the gene expression.