The sera of 100 Colombian individuals of African origin living in a malaria-endemic area of the Pacific coast were studied with regard to their capacity to inhibit Plasmodium falciparum cultures in vitro. Antimalarial antibody levels determined by indirect immunofluorescence were higher in the group of infected individuals than in the noninfected individuals, and inhibitory activity assessed by the inhibition of parasite incorporation of 3H-hypoxanthine in vitro was present in the sera of both the infected and noninfected patients. We believe that the noninfected patients were probably immune. The sera of some of the infected patients had high inhibitory capacities for the P. falciparum FCB-1 isolate. When the inhibitory effects of some of the sera were tested by using four parasite isolates from different regions of the world, striking differences among them were found.