Three cases of chloroquine-resistant malaria were studied in persons who contracted Plasmodium falciparum while exposed to infection by mosquito bite for a limited time in the Paillin District of Western Cambodia. The basis for the conclusion that the malaria did not respond normally to chloroquine was that 1) attacks occurred following chloroquine prophylaxis, and 2) relapses occurred following normal therapeutic regimens with the same drug. In one case, relapse occurred twice in the presence of plasma levels generally considered to be therapeutic.
One of the cases appeared to be resistant to pyrimethamine also. This point was not studied in the other two cases because pyrimethamine was not used.
Far East Research Project, Laboratory of Parasite Chemotherapy, National Institutes of Health, U. S. Public Health Service, Institute for Medical Research, Kuala Lumpur, Malaya.
Division of Malariology and Filariasis, Institute for Medical Research, Kuala Lumpur, Malaya.