A double-blind, randomized phase I clinical trial was carried out to compare Fansimef (a fixed-dose combination of mefloquine, sulfadoxine, and pyrimethamine) with sulfadoxine and pyrimethamine (Fansidar) for safety and tolerance. Twenty adult male Brazilian subjects from malaria endemic areas were studied for a period of 66 days, which included 2 days before and 63 days after drug administration.Both drugs were well tolerated and safe, as seen from the absence of drug-induced changes in the various laboratory, haematological, and biochemical parameters measured. Fansimef produced a complete clearance of parasites on day 3, with an "S" type response in one subject who had blood smears which were positive for Plasmodium falciparum on day 0. Two subjects in the sufladoxine-pyrimethamine group also had P. falciparum infections on day 0; the parasitaemia was cleared on day 2 in one of these subjects and on day 3 in the other, but an early RI response (recrudescence) was observed in the former case. Relapses due to P. vivax occurred in both groups.Side-effects due to Fansimef included mild dizziness, nausea, and vomiting. The incidence of dizziness and nausea was similar in the sulfadoxine-pyrimethamine group. In both groups, these side-effects were mild, short-lived and did not require specific treatment. Thus, Fansimef in an oral dose of three tablets (total of 750 mg mefloquine (base) plus 1500 mg sulfadoxine plus 75 mg pyrimethamine) was found to be well tolerated and safe.