The effects of sulfamonomethoxine, chloroquine and sulfamonomethoxine with pyrimethamine upon 66 cases of plasmodium falciparum and 28 cases of plasmodium vivax in Laos, were examined comparatively by the response to asexal parasitemia. Drugs were administrated to the three groups (A, B and C) as following mode : A. sulfamonomethoxine ; the first day : 40 mg. per Kg. of body weight the second day : 20 mg. per Kg. of body weight the third day : 20 mg. per Kg. of body weight B. chloroquine sulphate ; chloroquine base 1, 500 mg. C. sulfamonomethoxine with pyrimethamine ;sulfamonomethoxine 20 mg. per Kg. of body weightpyrimethamine 1 mg.per Kg. of body weight Results ; Chloroquine resistant patients were found and the response of the patients receiving sulfamonomethoxine to asexal parasitemia is much more effective than one of case with chloroquine. A dramatic effect was shown to the group treated with sulfamonomethoxine plus pyrimethamine. Asexal parasitemia disappeared from all patients. However, it should be notice that pyrimethamine resistant strain may appear. Judging from the above results, it can be emphasized that sulfamonomethoxine is useful as the primary drug against malaria under the keep of sulfamonomethoxine with pyrimethamine.