The first human retroviruses have been discovered during the past seven years. They cause two diseases which involve disturbances of the growth of the T4-lymphocyte. This target cell type, which is central to the regulation of the immune system is induced by human T-lymphotropic virus type I (HTLV-I) to excessive proliferation (leukaemia) and by HTLV-III/LAV (lymphadenopathy associated virus) to premature death (acquired immune deficiency syndrome [AIDS]). Both also seem to be indirectly involved in several other disorders. The genetic structures of these retroviruses and the mechanisms by which they usurp host-cell functions are novel among retroviruses. The continuous increase in the number of AIDS cases for whom no effective therapy is currently possible mandates attempts at developing primary prevention by a vaccine. Based on past attempts at developing vaccines against retroviruses, the most feasible configuration will be the glycoprotein linked to its transmembrane protein. Any virus preparation containing nucleic acids could be considered less safe. Potential problems exist in that there is extensive heterogeneity among various HTLV-III isolates, particularly in the env-gene. This fact and the known relationship of HTLV-III to some Lentiviruses suggest that functional antigenic variation could be encountered. The methodology of developing a vaccine against the retroviruses causing AIDS should also be helpful in designing vaccine strategies against human leukaemia and lymphomas caused by other members of this virus family.