Three quassinoids from the roots of Eurycoma longifolia Jack were evaluated for antimalarial activity against nine Plasmodium falciparum isolates obtained from patients infected with chloroquine-resistant malaria. The results indicated that eurycomanol, eurycomanol 2-O-beta-D-glucopyranoside, and 13 beta, 18-dihydroeurycomanol possessed antimalarial activity with IC50 values of 1.231-4.899 microM, 0.389-3.498 microM, and 0.504-2.343 microM, respectively, compared with 0.323-0.774 microM for chloroquine.