We studied in vitro susceptibility of 79 African strains of Plasmodium falciparum to artemisinin, chloroquine, quinine, mefloquine and halofantrine and the potential cross-resistance among the drugs. Most strains presented an 50% inhibitory concentration (IC50) of artemisinin below 26 nmol/liter. A positive correlation between IC50S of artemisinin and those of quinine, mefloquine and halofantrine was observed. Cross resistance between artemisinin and other antimalarials may limit its use as a replacement drug.