An IgM monoclonal antibody (MAb) against a carbohydrate epitope present in Trypanosoma cruzi trypomastigote excretory-secretory antigens and expressed by different developmental stages of the parasite (epimastigote, trypomastigote and intracellular amastigote) was linked to a solid phase matrix and used as an antigen-capture antibody. Human serum complexes containing the epitope were then detected by using specific secondary antibodies against human immunoglobulin isotypes. Results of detection of IgM, IgG, and IgA serum complexes (SC) containing a T. cruzi polypeptide epitope showed that SC could be detected in 69% of the 13 Chagasic acute phase sera studied with IgG, in 84% with IgM, and in 75% with IgA. Only 16% (IgG-SC), 8% (IgM-SC), and 10% (IgA-SC) of chronic sera from 50 patients were positive. No patients with toxoplasmosis or rheumatoid factor were positive. Of the 11 leishmaniasis sera studied, four had IgG-SC, two had IgA-SC, and five had IgM-SC. Of the eight Yanomamo Indians infected by Onchocerca volvulus, three were found to have IgG-SC, two had IgM-SC, and two had IgA-SC. Thirteen sera from healthy individuals living in an endemic area were also studied. One subject had IgG IgM and IgA-SC. The results presented in this study show for the first time, the specific detection of IgM, IgG, and IgA immune complexes using a MAb against T. cruzi. The presence of the epitope in association with IgM antibodies in sera from patients with the acute phase of the disease suggests that this antigen(s) carrying the epitope that reacts with the MAb could be a marker(s) of active infection. In addition, the specificity of the serum complex capture assay allowed the detection of Chagas' disease in two different endemic areas (Argentina and Venezuela).