A cDNA encoding human O6-methylguanine-DNA methyltranferase (MGMT) was cloned from Hela S3 cells and the sequence is identical with the published data. The MGMT cDNA was inserted into the expression vector pET-21a and transformed into E. coli BL21 (DE3). A 24 kD protein was expressed after IPTG induction. Essays using lethal dose of alkylating agents indicate that expression of MGMT protein can repair the DNA mutations of the recombinant bacteria produced by alkylating agents.