Two clone lines (Dd2 and 3D7) of Plasmodium falciparum were cultivated continuously in human erythrocytes at 37 degrees C in RPMI 1640 medium with human serum and subjected to 6% sorbitol treatment 2 times in order to obtain highly synchronized cultures. The second generation parasites after the treatment were diluted with human RBC to be a suspension of P. falciparum-human RBC at 2.5% hematocrit and 0.5% parasitemia, and 2 microCi/ml of 8-3H-hypoxanthine was added. Isotopic microtest was employed to detect the antimalarial activity for 20 new compounds. Results revealed that the 20 compounds showed no anti-malarial activity, while the control drugs, chloroquine and quinine, exhibited high efficacy, indicating that the isotopic microtest is a stable and reproducible assay for screening new antimalarials.