Between 1981 and 1983, in vivo and in vitro studies were conducted in Haiti to assess the responsiveness of Plasmodium falciparum to chloroquine. The standard tests successfully performed included 92 WHO standardized in vivo field tests and 160 in vitro tests (64 macrotests, 33 microtests, and 63 48-hr tests). No clearcut evidence of chloroquine resistance was detected. In 3 in vivo and 5 in vitro tests, a decreased susceptibility to the drug was suggested, but these isolated findings failed to be corroborated by parallel alternate tests. In addition, during the initial trial of an alternate monitoring system, 339 simplified 7-day in vivo tests were successfully performed, with no suggestion of resistance detected. This simplified 7-day in vivo test potentially represents an efficient low cost method for monitoring drug resistance in many developing countries.