During the early months of 1985 and 1986, 408 non-immune British soldiers undertook training in the jungles north of Kota Tinggi, in southern Malaysia. In this geographical location, where malaria transmission is now light, a combination of strict personal antimosquito measures plus proguanil 200 mg daily produced effective, safe protection, with only a single case of vivax malaria occurring. Given the limited malaria risk, however, the results of this study should be extrapolated only with caution to other areas of Malaysia.