The biological properties of the ruminant animal lentiviruses, visna and caprine arthritis-encephalitis viruses, closely resemble those of their human counterparts, the human immunodeficiency viruses (HIV). All of these viruses are morphologically identical and are disseminated from host to host in nature during exchange of body fluids. Artificial conditions that favor excess exchange of such fluids precipitate epidemics by these viruses. The strategy of replication of the animal viruses in tissue culture and in vivo are very similar to that of the human virus. Virus replication is highly productive in tissue culture and leads to cytopathic effects characterized by fusion. In vivo, the rate of virus replication is restricted and lesions, suggestive of an immunopathological origin, develop after prolonged periods of subclinical infection. Similar to the animal viruses, the human viruses have a tropism for macrophages in vivo, and this leads somehow to a loss of T helper lymphocytes and proliferation of cytotoxic lymphocytes. In addition, the viruses are highly neurotropic and this results in acute fulminating disease in neonatal hosts and chronic encephalopathy in adults. Both animal and human viruses cause persistent infections and have similar strategies for eluding host immune responses. These include sequestration of neutralizing epitopes, induction of low titers of neutralizing antibodies, and antigenic drift during persistent infection. Despite close homology between genetic sequences of HIV-I and -II, these two viruses seem to have as much biological disparity from each other as does visna virus from caprine arthritis-encephalitis virus. The latter two viruses induce neutralizing antibodies that are highly strain specific and show no cross protection.(ABSTRACT TRUNCATED AT 250 WORDS)