A retrospective examination was made of archival data on the response of Plasmodium malariae infections in humans to chloroquine. The clearance time for P. malariae was longer than that for P. falciparum and P. vivax. Of 100 P. malariae-infected patients treated with 1,500 mg of chloroquine given over 3 days, 15 had detectable parasites for 7 days, 4 for 10 days, and 1 for 15 days after treatment. Of 17 patients treated intramuscularly with 450 mg of dihydrochloroquine, parasites persisted in 1 patient for 11 days. Of patients with chloroquine-sensitive P. falciparum. 44 cleared parasites by 6 days after treatment; 37 patients with P. vivax infections cleared parasites by day 5. The confirmation of chloroquine resistance may depend on the adaptation of isolates to nonhuman primates in which controlled drug trials can be made.