The antiplasmodial effect of a single dose treatment with a sulfadoxine/pyrimethamine combination as compared to a sulfalene/pyrimethamine combination against falciparum malaria was assessed in semi-immune populations in Burma in early 1980. Parasite clearance rates on Day 7 after treatment were 99.2% for the sulfadoxine/pyrimethamine combination and 98.6% for the sulfalene/pyrimethamine combination for all age-groups. The earlier recrudescence rates within one month were 3.7% and 9.2% respectively, while the later recrudescence rates between 1 and 2 months were 9% and 8.3% respectively. Hence, both combinations were equally effective for treatment of falciparum malaria as no significant difference in the parasite clearance rates was observed. However, the earlier recrudescence rates showed a significant difference with a higher rate for the sulfalene/pyrimethamine combination. This is thought to be due to the shorter half-life of sulfalene compared to sulfadoxine and to its being unable therefore to suppress the falciparum infections for as long a period as sulfadoxine. But there was not much difference in the later recrudescence rates. These combinations have a stimulating effect on the production of falciparum gametocytes; and, in order to minimize transmission, an effective gametocytocide such as primaquine should be given along with them, as well as with the chloroquine/pyrimethamine combination, in areas with efficient malaria vectors. Recrudescence rates and gametocyte rates were highest among children in the 1-4 years age-group and this could be attributed to their lower level of acquired immunity compared to the older children and adults. Vivax malaria was also found to be effectively suppressed for about 4 weeks with both combinations.(ABSTRACT TRUNCATED AT 400 WORDS)