The anti-plasmodial activity of Ailanthus excelsa stem bark was investigated. The methanolic extract inhibited in vitro growth of chloroquine-sensitive (D10) and resistant strains (W2) of Plasmodium falciparum (IC50 4.6 and 2.8 microg/ml, respectively). The effect was retained in the chloroform fraction (3.1 and 2.1 microg/ml, respectively). The anti-plasmodial activity could be ascribed to the impairment of haemoglobin degradation through the inhibition of plasmepsin II activity (IC50 of 13.43+/-1.74 microg/ml) and of the haem detoxification to haemozoin.