In a chloroquine-resistant Plasmodium falciparum endemic area of Hainan Island, China, 1·0 g oral mefloquine produced a radical cure in 47 of 48 semi-immune patients. A comparison between patients treated with mefloquine and with oral qinghaosu showed a more rapid clearance of parasitaemia with qinghaosu (68·2±21·4 h vs103·1±18·0 h) and a greater inhibition of in-vivo trophozoite development. An advantage of mefloquine is the effectiveness of a single oral dose, whereas the advantages of qinghaosu are the speed of onset of action and inhibitory effect on parasite maturation.