Rhesus monkeys (Macaca mulatta) gamma delta T cells were identified using a monoclonal antibody. The relative representation of gamma delta T lymphocytes in the peripheral blood, lymph nodes, and spleen resembles that of Homo sapiens. The analysis of function and specificity revealed further significant similarities between the simian and human gamma delta T-cell systems. Since both human and monkey gamma delta T lymphocytes can effectively lyse cells infected with immunodeficiency viruses, it is possible that the primate gamma delta T-cell systems contribute to antiviral immunosurveillance.