For the eradication of malaria from hyperendemic regions of tropical Africa it is apparent that use may have to be made of antimalarial drugs, administered individually on a census basis, in addition to measures directed against the mosquito. The suppressive activity of existing compounds among individuals having different degrees of immunity is well established, and trials among large groups of people have been conducted with single drugs and with combination of drugs. In the large-scale trials carried out in Tanganyika and described in this paper, such a combination, containing amodiaquine for schizontocidal effect and primaquine as a gametocytocide, was administered to three distinct population groups of more than 5000 at differing intervals of time, in order to determine the ability of this combination to interfere with transmission in the absence of other malaria control measures. It was found that treatment of 93% of the population at intervals of one or two weeks resulted in a reduction of the malaria indices to a very low level but such success was not obtained when the combination of drugs was administered every four weeks, although in the area concerned population coverage was less satisfactory owing to migration.