Summary and Conclusions A strain of Plasmodium falciparum from the Magdalena Valley, Colombia, South America, which is resistant to chloroquine, has been shown to be resistant also to amodiaquine (Camoquin®) and to hydroxychloroquine (Plaquenil®), which are other members of the 4-aminoquinoline group. The resistant quality persisted after mosquito passage. The infections responded to quinine and to mepacrine, but relapses occurred. In areas where there is resistance to one of the above 4-aminoquinolines, it would appear unwise to depend upon another of the group for treatment, especially in acute cases; until evidence to the contrary is found, it seems desirable to use some other drug, perhaps quinine or mepacrine.