Oral treatment with clindamycin (5 mg/kg twice a day, for five consecutive days) was studied in patients with uncomplicated falciparum malaria in Acre, Brazil, an area with multiresistant Plasmodium falciparum. Parasitaemia ranged between 12 and 79560/microliters of blood admission. Thirty-five out of 44 patients admitted to the study could be followed up for 28 days. Only two patients showed parasitaemia six days after admission, and no asexual parasites were observed by day seven. Twenty-eight days after admission all patients were cured. Of the nine patients withdrawn from the study, five were lost during follow up and four needed different treatment (quinine 15 mg/kg twice a day, for ten days) because clinical symptoms did not improve within 60 h after admission. These patients had experienced their first attack by P. falciparum. In individual cases oral clindamycin can be used as an alternative treatment in semi-immune patients with uncomplicated falciparum malaria from an area where multiresistant parasites frequently occur. However, because of the slow response in all cases described here, and the risk of development of resistance if clindamycin is used alone it cannot be recommended as monotherapy in non-immune patients.