1. An investigation is described in which eighty cases of acute malaria treated with prontosil are compared with sixty-eight cases treated with quinine bihydrochloride. 2. It is found that prontosil is not as efficient as quinine in P. falciparum malaria. 3. Prontosil is still less effective in P. vivax and P. malariae malaria. 4. Prontosil is not an efficient gametocide in either P. falciparum or P. vivax malaria. Mosquitoes were fed on “crescent” carriers who had been given prontosil for 7 days and were found to be readily infected. 5. No toxic effects were noted. 6. It is concluded that prontosil, though it has some lethal action on malaria parasites, especially P. falciparum, has no place in the practical treatment of malaria owing to its low efficiency, possible toxicity, and relatively high cost.