Infective forms of Trypanosoma cruzi, the parasite that causes Chagas' disease, express on their surface an enzyme denominated trans-sialidase (TS). The present study was designed to evaluate the naturally acquired immune responses to a bacterial recombinant protein representing the catalytic domain of TS in chronically infected chagasic individuals. The cellular immune response was measured by in-vitro T-cell proliferation and by interferon (INF)-gamma, interleukin (IL)-4 and IL-10 production in response to a whole-parasite homogenate and the recombinant protein. The peripheral blood mononuclear cells of 78.6% of 28 chagasic patients responded to the recombinant protein as estimated by T-cell proliferation. With respect to cytokine production, 88% of the cells of the chagasic individuals produced IFN-gamma on stimulation with the recombinant protein. In contrast, IL-4 or IL-10 were minimally produced in response to TS. The cellular immune response was specific because most healthy individuals never exposed to T. cruzi failed to react with this recombinant protein. The plasma of 71.4% or 100% of chagasic patients had IgG antibodies as determined by ELISA or by the presence of TS inhibitory antibodies, respectively. We conclude that the catalytic domain of TS is recognized by IFN-gamma producing type 1 cells and antibodies in a large proportion of patients infected with T. cruzi.