Chagas' disease, leishmaniasis, and Trypanosoma rangeli infection are endemic and their distributions overlap in vast regions of South and Central America. Serological cross-reactivities can confuse epidemiological studies of these infections, and their differential diagnosis has been assigned a high priority by the World Health Organisation. A lectin-affinity-purified, 90,000 molecular weight glycoprotein (GP90) is present in the known principal strains (zymodemes) of Trypanosoma cruzi and absent from Leishmania and T rangeli. Patients with T cruzi infection have antibody to GP90, whereas patients with leishmaniasis do not and the two infections can be distinguished in an ELISA system using this antigen. In a mouse model, the same test can differentiate between T cruzi and T rangeli infections. Antigens purified by affinity chromatography clearly provide a practical basis for very precise, even strain-specific, diagnostic tests.