338 subjects occupationally exposed to high levels of malaria transmission were randomly assigned to three groups. Group A received one tablet of mefloquine (250 mg) and one tablet of sulfadoxine-pyrimethamine once a week, group B received two tablets of sulfadoxine-pyrimethamine once a week and group C received one tablet of sulfadoxine-pyrimethamine (each containing sulfadoxine 500 mg and pyrimethamine 25 mg) twice a week. Blood films for malaria parasites and filter paper strips for serological study were taken before and at 5 and 12 months after the chemoprophylaxis. The advantages and inherent limitation of the slide positivity rate and the usefulness of geometric mean reciprocal titre for the assessment of the efficacy of the chemoprophylaxis were discussed.