The overexpression of mutant forms of O(6)-methylguanine-DNA-methyltransferase (MGMT), resistant to the MGMT inhibitor O(6)-benzylguanine (BG), protects hematopoietic cells from the toxicity of combined BG plus O(6)-alkylating agent chemotherapy. To evaluate the feasibility of this approach for clinically relevant O(6)-alkylating agents, combined therapy with BG and two chloroethylnitrosourea-type drugs, ACNU or BCNU, or the triazene derivative temozolomide (TMZ) was investigated in a murine bone marrow transplant model allowing transgenic expression of the highly BG-resistant MGMT(P140K) mutant. Whereas 20/20 control animals transplanted with nontransduced cells died of progressive myelosuppression during therapy, nearly all animals transplanted with MGMT(P140K)-transduced cells survived treatment with BG/ACNU (12/15), BG/TMZ (10/10), or BG/BCNU (5/5). In surviving animals, hematological parameters improved during chemotherapy and pretreatment levels were reestablished during or shortly after therapy. All animals showed enrichment of transgenic granulocytes (range: 15- to 101-fold) and lymphocytes (range: 16- to 55-fold) in peripheral blood, bone marrow, and spleen. No significant differences were observed between individual treatment groups. Serial transplants demonstrated protection in secondary recipients and confirmed the transduction of transplantable stem cells. Thus, these data demonstrate efficient protection from hematotoxicity and substantial enrichment of transgenic cells following MGMT(P140K) gene transfer and treatment with different O(6)-alkylating drugs.