The in vitro chloroquine potentiating action of racemic chlorpheniramine was assessed by a semi-microtest against chloroquine-resistant African isolates of Plasmodium falciparum. At the chlorpheniramine concentra-tions of 312 and 625 nM, chloroquine resistance was reversed in 4 and 11 of 14 iso-lates, respectively, with a diminution of their 50% inhibitory concentration (IC₅₀) values for chloroquine to below 100 nM. No effect was observed against the chloroquine-susceptible clone. The results of the study suggest that chlorpheniramine may be a promising chloroquine resistance modulator.