A 48 h in vitro test of the efficacy of artemisinine, dihydroartemisinine, artemether, mefloquine and chloroquine was carried out against 3 chloroquine-resistant strains of Plasmodium falciparum, strains K1 and T996 from Thailand and LS21 from India. A sigmoid Emax model was fitted to all in vitro inhibition data for each combination of drug and strain. Strains K1 and LS21 were strongly resistant to chloroquine, whereas T996 was partially resistant. Artemisinine, dihydroartemisinine and artemether were active against all strains, with complete growth inhibition at 10(-7) M. Artemether and dihydroartemisinine were both more potent than artemisinine, with 50% effective (EC50) values of 0.57-1.6 nM and 0.36-3.1 nM respectively; the EC50 of artemisinine was 1.5-6.1 nM for the 3 strains. The EC50 values for mefloquine were 46-185 nM. At higher concentrations, strains K1 and LS21 were fully inhibited, while with strain T996 mefloquine did not fully inhibit even at the highest concentration, 1.28 x 10(-6) M. It is concluded that artemisinine and its derivatives were highly effective against the 3 chloroquine-resistant strains, one of which showed borderline resistance to mefloquine.