Chloroquine sensitivity of isolates of Plasmodium falciparum adapted to in vitro culture.

Thirteen isolates of Plasmodium falciparum obtained from cases of malaria imported into the Netherlands and established in culture were tested for their sensitivity to chloroquine. Reproducibility of the test results depended on the exposure of a standardized number of parasites in culture to the drug. The maximum activity of chloroquine was obtained when medium with the drug was added to parasite cultures twice at 24 hour intervals. The result of drug action over a period of 48 hours was estimated best when parasites were counted 72 hours after the commencement of the test. Sensitivity to chloroquine could not provide a basis for the characterisation of strains.