The specificity of a circulating antibody observed in American trypanosomiasis and reacting with endocardium, blood vessels, and the interstitium of striated muscle (EVI factor) was evaluated in the indirect fluorescent antibody test with 60 sera from patients with malaria, leishmaniasis, echinococcosis, amebiasis, African trypanosomiasis, toxoplasmosis, and trichinosis, collected from areas where Chagas' disease is not endemic. Two sera, 1 from a patient with Plasmodium falciparum malaria and 1 from a patient with a relapse pretreatment post kala-azar dermal leishmaniasis, were positive for the EVI factor. In the leishmaniasis group, 3 of 8 sera reacted with 0ovine, murine, and human skeletal muscle. In this reaction, which differs from the EVI test, the sarcolemma and the intracellular structures were stained.