Antiparasitic activity of biochanin A, an isolated isoflavone from fruits of Cassia fistula (Leguminosae). The fractionation through bioguided antileishmanial activity of the dichloromethane extract of Cassia fistula fruits (Leguminosae) led to the isolation of the active isoflavone biochanin A, identified by spectroscopic methods. This compound showed 50% effective concentration (EC(50)) value of 18.96 microg/mL against promastigotes of Leishmania (L.) chagasi. The cytotoxicity of this substance against peritoneal macrophages resulted in an EC(50) value of 42.58 microg/mL. Additionally, biochanin A presented an anti-Trypanosoma-cruzi activity, resulting in an EC(50) value of 18.32 microg/mL and a 2.4-fold more effectiveness than benznidazole. These results contribute with novel antiprotozoal compounds for future drug design studies.