Susceptibility of Plasmodium falciparum to chloroquine in Liberia, West Africa: in vitro tests for 24 and 48 hours.

In vitro tests of Plasmodium falciparum susceptibility to chloroquine were performed in May 1982 in Yekepa area, northern Liberia where several previous in vivo investigations had shown full susceptibility. The isolates from 19 subjects were studied. 18 Rieckmann 24-h micro tests were successful. Effective chloroquine concentrations by probit analysis were 0.3 microM for 50% inhibition (EC50) and 1.2 microM for 99% inhibition (EC99). 18 of the 48-h tests were successful. 10 isolates were inhibited by 0.75 microM chloroquine and another 8 by 2.5 microM.