A simplified in vitro technique was evaluated for reliability and reproducibility in detecting chloroquine-resistant strains of Plasmodium falciparum in Central Thailand. In 55 of 57 subjects the parasites were resistant to chloroquine both in vitro and in vivo. Two subjects who were cured with conventional chloroquine administration had parasites which were resistant in vitro. The major technical limitations of the in vitro test were growth failures with high parasite densities and with immature trophozoites. With these limitations the in vitro technique gave a reliable and reproducible qualitative index of chloroquine resistance in falciparum malaria.
U.S. Army Medical Component, SEATO, APO San Francisco 96346.