A randomized study on the effect of the following four treatment regimens on Plasmodium falciparum parasitaemia was carried out on 200 asymptomatic schoolchildren in Maputo, Mozambique: chloroquine (25 mg/kg body weight), amodiaquine (25 mg/kg), sulfadoxine-pyrimethamine (25 mg/kg and 1.25 mg/kg), or amodiaquine (25 mg/kg) + sulfadoxine-pyrimethamine (25 mg/kg and 1.25 mg/kg) administered on the third day of the study. The results of in vivo tests indicated that 94% of the infections were resistant to chloroquine, 76% to amodiaquine, and 16% to sulfadoxine-pyrimethamine. The cure rate with amodiaquine + sulfadoxine-pyrimethamine was 100%, which was not significantly different from that with sulfadoxine-pyrimethamine alone; the latter regimen was the most rapidly acting of the treatments studied. It is concluded that amodiaquine is not an appropriate substitute for chloroquine, but that the effect of the combination amodiaquine + sulfadoxine-pyrimethamine may be superior to that of sulfadoxine-pyrimethamine alone, although this requires further study