Establishment of immunocompetent cell mediated anti-tumor immunity is often mitigated by the myelosuppressive effects during administration of chemotherapy. We hypothesized that protecting these immune cells from drug induced toxicities may allow for the combined administration of immunotherapy and chemotherapy, Using a SIV-based lentiviral gene transfer system we delivered the drug-resistant variant P140KMGMT into the immunocompetent cell lines NK-92 and TALL-104, and the myelogenous leukemia cell line, K562, which is a target for both NK-92 and TALL-104 cells, Genetically engineered immunocompetent cells developed significant resistance to temozolomide compared to non-modified cells, and genetic modification of these cells did not affect their ability to kill K562 cells We then evaluated the effectiveness of drug-resistant immunocompetent cell mediated killing of tumor cells in the presence and absence of chemotherapy During a chemotherapy challenge the cytotoxic activity of non-modified immunocompetent cells was dramatically impaired However, when combined with chemotherapy, genetically-modi fled immune cells retained their cytotoxic activities and efficiently killed non-modified target cells These results show that engineering immunocompetent cells to withstand chemotherapy challenges can enhance tumor cell killing when chemotherapy is applied in conjunction with cell-based immunotherapy (c) 2009 Elsevier Inc. All rights reserved.