We examined the requirement of Tropanosoma cruzi protein tyrosine phosphorylation for parasite entry into mammalian cells and analyzed the profile of phosphorylated proteins in infective trypomastigotes. Treatment of metacyclic or tissue culture trypomastigotes with genistein, an inhibitor of protein tyrosine kinase activity, significantly inhibited invasion of cultured HeLa cells. A soluble factor, contained in HeLa cell extract and absent in the extract ot T. cruzi-resistant K562 cells, greatly enhanced phosphorylation levels of a 175-kDa protein (p175) in trypomastigotes. Genistein inhibited p175 tyrosine phosphorylation. P175 was undetectable in noninvasive epimastigotes. The phosphorylation-inducing activity of HeLa cell extract was abrogated by adsorption with metacyclic trypomastigotes but not with epimastigotes or when it was mixed with recombinant protein J18, which contains the entire peptide sequence of gp82, a metacyclic stage-specific surface glycoprotein implicated in target cell invasion. These data suggest that, in metacyclic trypomastigotes, gp82 is the signaling receptor that mediates protein tyrosine phosphorylation necessary for host cell invasion.