Abstract: An invitro microtechnique was developed for the deter mination of antimalarial activity of drugs on Plasmodium falciparium strain FCC₁. Plastic plates each with 40 wells were used. Each well (6.7 mm in diameter and 10 mm in depth) held 180 μl of RPMI 1640 culture medium containing 2.5% RBC (infEction rate about 1%) and 20 μl of glucose saline containing drugs to be tested. The cultures were incubated at 36～37℃ in candle-jar for 48 hrs., according to Trager & Jensen's method, without changing the medium during the incubation period. In the control, the parasites multiplied with a rate increasing from 1.05±0.10% to 3.75±0.89%. Using Finney's method the ED₅₀ of the tested drug was calculated as a dose which causes 50% reduction of the parasite as compared with control. As shown in Table 3 the mean ED₅₀ of chloroquine was 4.40±0.82ng/ml(5.59～3.36ng/ml). This method is considered to be useful and reproducible for testing antimalarials directly with human malaria parasites.