Diospyrin and its four synthetic derivatives were tested in vitro against intracellular amastigotes of Leishmania donovani and Trypanosoma cruzi in macrophages and extracellular Trypansoma brucei brucei bloodstream form trypomastigotes. Diospyrin was inactive against L. donovani but had ED(50) values of 27 and 50 mu M, respectively, against T. cruzi and T.b. brucei, The dimethyl ether derivative was more active than the parent compound against T. cruzi and T.b. brucei, but the hydroquinonoid form was the most active antiparasitic agent with ED,, values of 2.2 and 0.7 mu M, respectively, against L. donovani and T.b. brucei.