SummaryThirty-three children aged 6 months to 7 years from an area with multidrug-resistant Plasmodium falciparum strains were treated with 25 mg/kg body weight of mefloquine hase as a single oral dose. They were followed-up using the modified 28-day W H O extended field test. The parasite isolates from these patients were cultured in vitro with different concentrations of mefloquine. All children were parasite-negative by day 4, and 3 T remained so throughout the period of observation. Two patients who were parasitaemic o n days 1 6 and 2 8 were successfully treated with a sulphadoxine/pyrimethamine combination. Parasitological and clinical responses were well correlated. The mean parasite clearance time was 65 i 50.2 hours. A mefloquine concentration of 64 pmol/well inhibited schizont growth and the EC,, and EC,, were 5.5 and 54 pmol/well (1.1 and 10.8 pmol/l blood) respectively. This indicates reduced parasite susceptibility to the drug in vitro.keywords mefloquine, resistant Plasmodium falciparum strains