Human T cell lymphotropic virus types I and II (HTLV-I/II) are endemic in certain areas of the world. They cause two life-threatening diseases, adult T cell leukaemia/lymphoma and tropical spastic paraparesis. A vaccine is needed because in developing countries there are no other feasible preventive interventions against these diseases and in Western countries intravenous drug users at high risk for HTLV-I and HTLV-II infections and the health workers in contact with such populations must be protected. We have developed a rat model in which we observed variations of susceptibility to viral infection between inbred strains, the most susceptible being the Fischer F344, and the possibility of viral latency in the nervous system. We have prepared a recombinant adenovirus vector that expresses the HTLV-I envelope glycoprotein env in HeLa cells. A target human population in French Guyana, in which the prevalence rate reaches 5.6% in one ethnic group (Bonis), has been identified for possible intervention.