The macaque immunodeficiency syndrome has many parallels to AIDS in humans. Affected monkeys develop profound, prolonged T lymphocyte dysfunction and die of lymphomas or opportunistic infections. We recently isolated a virus that we call SIV from four sick macaque monkeys. The morphology, growth characteristics, and antigenic properties of this virus indicate that it is related to the causative agent of human AIDS. The pathogenicity of this newly isolated virus was tested in macaque monkeys. Five of six died between 127 and 352 days following inoculation. The animals developed a wasting syndrome and died with adenovirus pancreatitis and/or pneumonia and primary retroviral encephalitis. Immunological abnormalities in these animals included a decrease in circulating T4+ lymphocytes and depressed peripheral blood lymphocyte proliferative response to pokeweed mitogen. The SIV monkey model holds great promise for testing antiviral agents and for the development of vaccines against AIDS.