Diphyllin isolated from Haplophyllum bucharicum Litv. (Rutaceae), an endemic plant of Uzbekistan, displayed a moderate antiproliferative activity towards human monocytes (IC50 = 35.2 microM) and Leishmania promastigotes (IC50 = 14.4 microM), by a mechanism of action that involved interaction with macromolecules and resulted in cell cycle arrest in the S-phase and inhibition of protein synthesis. In the intracellular amastigote form of the parasite, diphyllin exerted a strong specific inhibitory activity (IC50 = 0.2 microM) resulting from the inhibition of parasite internalization within macrophages. This property was mainly due to modulation of macrophage phagocytosis and, to a lesser extent, it also involved interference with surface molecules of the promastigote membrane.