Artemisinin (Qinghaosu), a poorly soluble sesquiterpene lactone derived from the plant Artemisia annua Linn., and a number of more soluble, semi-synthetic derivatives are rapidly-acting blood schizontocides against Plasmodium berghei and P. yoelii nigeriensis. An oily suspension of artemisinin given s.c. is more effective than aqueous suspensions. The activity is retained against lines resistant to primaquine, cycloguanil, pyrimethamine, sulphonamides, mefloquine and menoctone, but a highly chloroquine-resistant line is much less sensitive. Artemisinin has no causal prophylactic, gametocytocidal or sporontocidal action. Dihydroartemisinin causes the pigment of P. berghei to clump, but in a different fashion from the pigment changes induced by chloroquine or quinine, reflecting a different mode of action of the sesquiterpenes from that of these other antimalarials.