The mechanisms of cytotoxic killing of various tumor cell lines and immunodeficiency virus-infected T cell lines by simian γδ T cells were examined. The lysis of the majority of the target cell lines by γδ effectors was calcium-dependent, indicating that cytotoxicity is mediated by the perforin/granzyme pathway rather than the Fas-FasL pathway, with the exception of Jurkat cells. The γδ T cells were able to suppress SIV replication as measured by the p27 ELISA and the suppression was contact-dependent. We further determined that the target cells were induced to undergo apoptosis by the γδ T cell effectors. These results contribute to our understanding of the function of simian γδ T cells and their similarities to human γδ T cells, and extend our knowledge on the cytotoxic mechanisms employed by γδ T cells in general.