During 1967 more than 300,000 American troops are expected to return to the United States from South Vietnam. Those who have engaged in combat in the central highland jungle have probably been exposed to virulent strains of Plasmodium falciparum malaria. These troops have been taking a tablet containing 300 mg chloroquine (base) and 45 mg primaquine (base) once weekly as chemoprophylaxis. Some are receiving a daily dosage of 25 mg of dapsone (diaminodiphenylsulfone) as a third chemosuppressive agent. On being rotated from Vietnam, each individual is issued a supply of the chloroquine-primaquine tablets with instructions to take one each week for eight weeks. He is warned not to use these combined tablets for the therapy of any clinical illness because of the potential hemolytic action of large doses of primaquine. Those troops participating in the field trial of dapsone are given a supply of the tablets and instructed to take