The action of 2:4-diamino-6:7-diisopropylpteridine on normal, proguanil- and sulphadiazine-resistant strains ofPlasmodium gallinaceum

2:4-diamino-6:7-diisopropylpteridine has an antimalarial index of 1:40, when tested againstPlasmodium gallinaceum. 2. 2:4-diamino-1′-methylindolo-(2′:3′-6:7)-pteridine showed no activity againstP. gallinaceumin doses tolerated by the chick. 3. Proguanil- and sulphadiazine-proguanil-resistant strains ofP. gallinaceumwere cross-resistant to 2:4-diamino-6:7-diphenylpteridine. 4. Proguanil- and sulphadiazine-proguanil-resistant strains ofP. gallinaceumshowed no cross-resistance to 2:4-diamino-6:7-diisopropylpteridine, but were slightly hypersensitive to this compound. I wish to thank Dr A. Bishop for her interest in this work, and for advice and criticism. The work has been financed by the Medical Research Council.