Sera collected at various time intervals from healthy Thai male subjects after the administration of the three oral formulations of dihydroartemisinin (Cotecxin) manufactured in the People's Republic of China, a formulation manufactured by Arenco n.v. Pharmaceutica in Belgium, and a formulation manufactured by the Faculty of Pharmacy of Mahidol University in Thailand) were investigated for their ex vivo blood schizontocidal activities against the K1 strain of Plasmodium falciparum. Blood schizontocidal activities of sera were evaluated using the maximum inhibitory dilution as a parameter. Sera obtained following the administration of the three formulations of dihydroartemisinin showed significantly distinct degree of ex vivo antimalarial activities. The differences may reflect the bioinequivalence between these three formulations of dihydroartemisinin. The ex vivo blood schizontocidal activity profiles generally coincided with plasma concentration-time profiles. Thus, the ex vivo model might be the useful tool for evaluating and comparing the bioequivalence of the interesting drugs especially where high-performance liquid chromatography with reductive electrochemical detection for drug analysis is not available. The effect of inoculum size of P. falciparum was shown in the ex vivo model as presented in the in vitro sensitivity test. To determine the effect of the inoculum size on the drug activity, the ex vivo model might be superior to the in vitro model since the pharmacokinetic profiles can be considered.