The in vivo persistence of gene-modified cells can be limited by host immune responses to transgene-encoded proteins. In this study we evaluated in a nonhuman primate model whether the administration of a nonmyeloablative regimen consisting of low-dose total-body irradiation with 200 cGy followed by immunosuppression with mycophenolate mofetil and cyclosporin A for 28 and 35 days, respectively, could be used to facilitate persistence of autologous gene-modified T cells when a transgene-specific immune response had already been established or to induce long-lasting tolerance in unprimed recipients. Two macaques (Macaca nemestrina) received infusions of T cells transduced to express either the enhanced green fluorescent protein and neomycin phosphotransferase genes or the hygromycin phosphotransferase and herpes simplex virus thymidine kinase genes. In the absence of immunosuppression, both macaques developed potent class I major histocompatibility complex-restricted CD8⁺ cytotoxic T-lymphocyte (CTL) responses that rapidly eliminated the gene-modified T cells and that persisted long term as memory CTL. Treatment with the nonmyeloablative regimen failed to abrogate preexisting memory CTL responses but interfered with the induction of transgene-specific CTL and facilitated in vivo persistence of gene-modified cells in an unprimed host. However, sustained tolerance to gene-modified T cells was not achieved with this regimen, indicating that further modifications will be required to permit sustained persistence of gene-modified T cells.