A microagglutination test using trypsin-treated and Coomassie blue-stained Trypanosoma cruzi epimastigote antigen was adapted for the diagnosis of Chagas' disease. When incorporated in the test, 2-mercaptoethanol treatment of chagasic sera had no influence on antibody titer. In contrast, titers in sera from patients with visceral leishmaniasis, African trypanosomiasis, and autoimmune disorders, subjected to similar treatment, showed remarkable decline. Accordingly, a lower cut-off point for Chagas' disease serological negativity could be taken resulting in a higher sensitivity (95.6%); the specificity was 94.7%. Similar specificities were obtained with Leishmania donovani chagasi and L. d. donovani antigens applied to homologous visceral leishmaniasis and heterologous Chagas' sera. Of 316 nonchagasic sera, only 3 with leptospirosis and 1 with leprosy showed seropositive titers prior to and after 2-mercaptoethanol treatment.