In 1982, 2 of 14 Plasmodium falciparum infections acquired in East Africa and diagnosed in Copenhagen were resistant to treatment with sulfadoxine plus pyrimethamine (Fansidar), while in 1983, 6 of 18 were so. The in vivo tests were supplemented by determinations of drug concentrations in serum, and 4 isolates from in vivo-sensitive cases and 6 from in vivo-resistant cases were selected for in vivo tests. These were performed in ordinary RPMI 1640 medium and in a medium with physiological p-aminobenzoic acid and folic acid concentrations. Pharmacokinetic aberrations were found to be of possible importance in only 2 of the in vivo-resistant cases. In vitro susceptibility to sulfadoxine was found to be uniformly low in all isolates. Testing with a combination of sulfadoxine and pyrimethamine in the medium with physiological concentrations of cofactors probably reflects the in vivo situation most accurately, but in all but 1 of the isolates studied in vitro the in vivo susceptibility to Fansidar would be predicted by in vitro susceptibility to pyrimethamine in either medium. The concentration of p-aminobenzoic acid in serum, quantitated by high performance liquid chromatography, was found to be subject to wide variation, and this may have implications for in vitro testing.