Pyrimethamine sensitivity in Plasmodium falciparum: determination in vitro by a modified 48-hour test.

Four strains of Plasmodium falciparum recently isolated in culture were assessed in vitro for their response to pyrimethamine. A simple modified 48-hour test was used, which showed two strains to be sensitive to the drug in vitro, while the other two were resistant at a very high level. In the two strains for which relevant clinical information was available the in vivo response to pyrimethamine was corroborated by the in vitro findings. This modified 48-hour test is thus useful for determining patterns of drug sensitivity in laboratory-adapted strains, and would be a valuable asset if found to be equally applicable under field conditions.