A 48 h in vitro test was conducted to compare the susceptibility of two strains of Plasmodium falciparum to chloroquine, the two enantiomers of chloroquine, desethylchloroquine and the new antimalarial drug pyronaridine. The five compounds similarly inhibited the chloroquine sensitive strain. However, desethylchloroquine was less active and pyronaridine was much more active than chloroquine and its enantiomers against the chloroquine resistant strain.