Eleven Thai isolates and one West African isolate of Plasmodium falciparum were tested for their susceptibility to the Chinese antimalarial drugs artemisinine (qinghaosu) and artemether. The isolates were cultivated by the Trager-Jensen candle-jar technique and exposed to the action of the drugs for 36-48 hours. Artemisinine inhibited growth of most isolates at 10(-7)-10(-8) mol/litre and artemether at 10(-8) mol/litre (with an initial parasitaemia of 0.5-1.0%). Slight variation in the sensitivity of different isolates was found, but there was no correlation between sensitivity to artemisinine or artemether and sensitivity to pyrimethamine, pyrimethamine/sulfadoxine, or chloroquine. The action of artemisinine and artemether was reduced when the initial parasitaemia of the treated cultures was raised.