The target molecules on the cell surface of Trypanosoma cruzi trypomastigotes reacting with lytic anti-alpha-galactosyl antibodies from chronic patients with Chagas' disease (Ch anti-Gal) have been purified by solvent extraction and identified as glycoconjugates migrating in the 74-96-kDa range (F2 antigen) and in the 120-200-kDa range (F3 antigen) on SDS-PAGE. The F3 antigen was tested for binding to Ch and normal human serum (NHS) anti-Gal and to MoAb 3C9. We observed that Ch anti-Gal and MoAb 3C9, but not NHS anti-Gal, bind strongly to the trypomastigote glycoconjugates. These antibodies, however, did not compete with each other for binding to F3 molecules, indicating that they are recognizing different epitopes. Binding of Ch anti-Gal to F3 antigen is abolished by treatment of these molecules with alpha- but not beta-galactosidase. Binding of 3C9 MoAb is abolished by treatment of F3 with sialidase. F2/F3 antigens absorbed Ch anti-Gal as well as lytic antibodies from total chagasic sera. These antigens also specifically discriminate between the serum reactivity of patients with active Chagas' disease and those of sera from cured patients, drug-treated patients with dissociated serology (positive conventional serology, negative trypanolytic activity), healthy individuals, and patients with several other infectious diseases. We also observed that F2/F3 antigens are anchored to the parasite membrane via glycosylphosphatidylinositol (GPI). The alpha-galactosyl epitopes recognized by Ch anti-Gal are present in a series of O-linked oligosaccharide chains in the mucin-like glycoprotein component of the complex.