Cell suspension cultures developed from Artemisia annua exhibited antimalarial activity against Plasmodium faldparum in vitro both in the n-hexane extract of the plant cell culture medium and in the chloroform extract of the cells. Trace amounts of the antimalarial sesquiterpene lactone artemisinin may account for the activity of the n-hexane fraction but only the methoxylated flavonoids artemetin, chrysoplenetin, chrysosplenol-D and cirsilineol can account for the activity of the chloroform extract. These purified flavonoids were found to have IC50 values at 2.4 - 6.5 × 10(-5)M against P. falciparum in vitro compared with an IC50 value of about 3 × 10(-8)M for purified artimisinin. At concentrations of 5 × 10(-6)M these flavonoids were not active against P. falciparum but did have a marked and selective potentiating effect on the antiplasmodial activity of artemisinin.