A 30-h in vitro susceptibility test of Plasmodium falciparum wild isolates to artemisinin, chloroquine, sulfadoxine/pyrimethamine and mefloquine was performed in Kibaha, Tanzania. A sigmoid Emax model was fitted to all data for each isolate and drug combination. Artemisinin and mefloquine exhibited 100% growth inhibition against all isolates tested (n=69–74). The EC50 values for artemisinin and mefloquine were 44 and 146 nM respectively. Chloroquine and sulfadoxine/pyrimethamine resistance was 30% and 13% respectively. Susceptibility parameters (EC50, 90, 95 and 99 values and s) varied between compounds and isolates indicating the different sensitivity of P. falciparum isolates. No correlation between susceptibility parameters of artemisinin and the other compounds was found. The high in vitro activities of artemisinin and mefloquine indicate their potential role for the treatment of multidrug-resistant malaria in Africa.