The primate T cell lymphotropic virus group comprises pathogenic and apathogenic agents found in human and simian hosts. Up to date, three types of the simian T cell lymphotropic virus/STLV and four types of the human T cell lymphotropic virus/HTLV have been isolated and characterized from non human primates and from human hosts respectively. We have not found evidences of STLV-1 infection among new world monkeys and besides fi ndings of HTLV-1 and HTLV-2 infection among brazilian mixed ethnic populations and Amerindians respectively, some unresolved HTLV inde- terminate-Western blot results prevailed among human groups of different ethnic background. Based on recent serologic detection, isolation and characterization of HTLV-3 and HTLV-4 among African populations in central Africa and additional unrefutable evidences of early human migration from Africa and Australia to the American continent previously of Asiatic population migration lead us to hypothesize that human descendents of mixed Amerinds and Africans or remaining Africans explain the very frequent presence of Western blot-indeterminate results for HTLV-1/2 that we and other groups have been detecting and also the unusual absence of HTLV-2 infection among some relatively homogeneous ethnic native human populations in the American continent.