A glycoprotein of 25,000 daltons (G25) purified from T. cruzi extracts is recognized by serum antibodies of Chagas' disease patients. These human antibodies were isolated by affinity chromatography and were used to demonstrate that G25 antigenic determinants are i) represented at the parasite surface, and ii) are expressed in all developmental stages of the parasite's life cycle, as well as in several T. cruzi strains. This antigen-antibody system may be useful for the diagnosis of Chagas' disease because antibodies to radiolabeled G25 are found in the serum of 96.5% of 173 chagasic patients from different endemic areas, but are not found in the serum from other individuals. Taken collectively, the data suggest that antibodies to G25 define highly conserved determinants of the species T. cruzi. Moreover, its remarkable immunogenicity to infected humans offers an opportunity to investigate the role of specific immunologic responses in the pathogenicity of Chagas' disease.