Introduction SN-11,437, a metanilamide (Fig. 1), was chosen for study of its pharmocological and antimalarial properties in Plasmodium vivax infections in man, because it is an effective prophylactic agent in Plasmodium gallinaceum infection in the chick, at one-fifteenth the dose needed to achieve prophylaxis with sulfadiazine, and because it has high activity in other avian malarias (1, 2). The unique efficacy of a few sulfonamides, notably sulfadiazine, in gallinaceum infections, which is characterized by an abundance of exo-erythrocytic parasites (3), has created special interest in related compounds since these forms of the parasite are refractory to quinacrine and quinine. Considerable indirect evidence also suggests that the persistence of pre-erythrocytic or tissue stages is responsible for relapses in vivax malaria. The marked qualitative superiority of SN-11,437 over sulfadiazine in avian infections led to the consideration that it might prove effective in vivax malaria, where sulfadiazine has only slight, if any, effect as a suppressive agent (4, 5) and does not lead to radical cure (5, 6).