Decrease of tumor cellular resistance to ACNU by retroviral mediated antisense RNA targeting 5′ region of O 6 methylguanine DNA methyltransferase(MGMT)mRNA was reported.It was found that after MGMT gene expression was depleted by antisense RNA,the tumor implanted in nude mice exhibited to be more sensitive to the killing effect of ACNU.It suggested that appropriate antisense RNA might deplete MGMT activity in human tumor cells and decrease drug resistance to ACNU.