Using [G- 3H]hypoxan-thine uptake as a radioactive indicator for the growth of malarial parasites, we measured the antimalarial activity of artemisinin (Qinghaosu, QHS) against FCMSU 1/Sudan strain (chlo-roquine-sensitive strain) and FCB K + strain (chloroquine-resistant strain) of Plasmodium falciparum in continuous culture in vitro. The 50% inhibitory concentrations (IC 50) for QHS against FCMSU 1/Sudan strain and FCB K + strain were 2.8 × 10 −8 and 3.0 × 10 −8 M, respectively. On the contrary, the response of the two strains to chloroquine was quite different. The IC 50 for chloroquine against FCMSU 1/Sudan strain was 5.6 ng/ml, whereas that for the FCB K + strain was 65.6 ng/ml. Therefore, QHS did not appear to exhibit any cross-resistance with chloroquine. If [2,8- 3H]adenosine was used as a radioactive precursor instead of [G- 3H]hypoxanthine for the determination of antimalarial activity, virtually identical results were obtained. Therefore, [2,8- 3H]adenosine can be used as an alternative to [G- 3H]hypoxanthine for the assessment of antimalarial action.