The efficacy of different doses of chloroquine in suppressing patent parasitaemia was investigated in 326 children two to 12 years old, living in six villages with holoendemic malaria. The children were given single doses (2, 3, 5-7 or 9-12 mg base kg-1) or a standard treatment over three days (25 mg base kg-1). Parasite prevalences were recorded after one, two, three, four, six and eight weeks. Complete clearance of Plasmodium falciparum trophozoites (TC) by day 7 was achieved by a dosage of 9-12 mg kg-1. By probit analysis of log dose response, 50% clearance (TC50) was established at about 1.5 mg kg-1, whereas a TC95 required 5.5 mg kg-1. The reappearance of patent parasitaemia was dependent on the dose of chloroquine given and on malaria transmission. After the standard dose treatment, only one re-infection in 56 children appeared within 21 days despite high sporozoite inoculation rates in the area. The dosage of 9-12 mg kg-1 yielded a hundredfold reduction of mean parasite density in the children if calculated over a four-week period. It may represent a suitable monthly regimen in a malaria control scheme in a holoendemic area with high P. falciparum sensitivity to chloroquine.