Leishmania infection causes marked down-regulation of interferon (IFN)-γ-induced gene activity in macrophages, but the mechanism of the blockade has not been fully denned. The IFN-γ signal transduction pathway was analyzed in Leishmania donovani-infected phorbol-differentiated U937 human promonocytic cells. IFN-γ stimulation induced marked phosphorylation of its own receptor (IFN-γR)-α chain. Phosphorylation of the receptor subunit was significantly inhibited after 24 h of infection with the parasite, apparently because of decreased amounts of the receptor subunit. Formation of the IFN-γR complex, as assessed by tyrosine phosphorylation and association of Jak2, was strongly inhibited in cells infected for 24 h. Inhibition of the IFN-γR complex formation correlated with inhibition of STAT1α binding to the IFN-γ response region. Pretreatment with purified parasite lipophosphoglycan before IFN-γ stimulation had no effect on tyrosine phosphorylation. Thus, inhibition of tyrosine phosphorylation of the IFN-γR-α chain and subsequent signal transduction are most likely due to the decreased amount of IFN-γR-α protein after infection.