ABSTRACT
A field study was conducted on the island of Mindoro, Republic of the Philippines in which over 800 persons were screened for malaria and approximately 8% were found positive. The in vitro microtechnique was used to test for sensitivity to chloroquine, amodiaquine, mefloquine and quinine in 20 slide-confirmed P. falciparum cases. Sixteen of these cases were also followed for in vivo chloroquine sensitivity. Four cases showed in vitro resistance to chloroquine; 2 also showed resistance to quinine. All showed in vitro sensitivity to mefloquine and amodiaquine. The results of in vivo test were consistent with either a sensitive (S) or R-1, resistant response to chloroquine. Taken together, the in vitro and in vivo chloroquine tests indicate 4 cases of chloroquine resistance at the R1 level.