We found that Trypanosoma cruzi trypomastigote cloned surface ligand (gp83 trans-sialidase) signals macrophages to up-regulate parasite entry by activating protein kinase C (PKC). Incubation of r-gp83 ligand with macrophages activates PKC and this activation is abolished when r-gp83 is depleted by immunoprecipitation with anti-r-gp83 antibodies, which recognize the secreted gp83 of trypomastigotes by immunoblotting. This activation is seen as early as 15 min with maximal activity at 60 min and correlates with the concentration of macrophage cell cytosol. Bisindolylmaleimide I, a PKC inhibitor, abolished the activation of PKC induced by r-gp83 ligand. Incubation of macrophages with r-gp83 ligand significantly enhanced the number of trypanosomes per cell. Bisindolylmaleimide I also inhibited the enhancement of trypomastigote uptake by macrophages induced by the r-ligand. These results demonstrate that T. cruzi uses a novel mechanism to signal cells in the process of trypanosome entry, via a secreted trypanosome ligand which signals macrophages through activation of PKC.