O6-Methylguanine-DNA methyltransferase (MGMT) is an important DNA repair protein that plays a key role in cancer chemotherapy by alkylating agents such as carmustine (BCNU) and Dacarbazine (DTIC). Therapy by BCNU and DTIC is reduced by dose-limiting hematological toxicity as a result of low MGMT repair activity in bone marrow cells. In this study, we have constructed a Moloney murine leukemia virus retroviral vector containing the human mgmt gene. High-titer retrovirus producer cells lines have been generated. Retroviral-mediated transfer of the human mgmt gene into murine multi-potent hematopoietic stem cells, FDCP-1, resulted in the expression of a high level of MGMT activity. In comparison with the control cells that were transduced with the parent vector, the MGMT-expressing clones were considerably more resistant to the cytotoxicity of the methylating agents, such as N-methyl-N'-nitro-N-nitrosoguanidine, N-nitroso-N-methyl-urea, and temozolomide, as well as the chloroethylating agents 1-(2-chloroethyl)-1-nitrosourea and BCNU. The protection provided by MGMT could be eliminated by the MGMT inactivator O6-benzylguanine. Thus, the principal lethal lesions produced by these alkylating agents in the murine hematopoietic stem cells and the MGMT deficiency in these cells can be complemented by retroviral-mediated gene transduction.