Effect of 6-sulfooxymethyl benzo(a)pyren (SMBP), an ester form of benzo (a)pyrene metabolite, on cytotoxicity and on change of DNA structure were investigated. Relative survival rate was linearly decreased within the range of 1.0$\mu$M SMBP. Glutathione S-transferase (GST) and vitamin C reduced the cytotoxicity of SMBP in dose dependent manner and it appears that GST detoxify damage by SMBP more effectively than Vitamin C. Supercoiled DNA was changed to relaxed and linear DNA by treatment of SMBP. The result suggest that SMBP nicks the plasmid and supercoil DNA can be cleaved to small DNA fragments. 6-Sulfooxymethyl benzo(a) pyren keeps B-DNA conformation because the transition of B-Z could be related to regulation of gene. When calf thymust DNA was incubated with SMBP, the major DNA adduct was identified as the deoxyguanosine. This DNA adducts may cause cytotoxicity and further mutagenicity.