The effect of 6-sulfooxymethyl benzo(a)pyrene (SMBP) on conformational changes of calf thymus DNA was investigated. As SMBP is a strong electrophile, the covalent binding of SMBP to DNA should distort three dimensional conformation of DNA at the binding sites. A formaldehyde-unwinding methods were used to determine the rate of DNA denaturation. The increase in absorbance at 251nm was detected by addition of formaldehyde following treatment with SMBP. SMBP changed supercoiled DNA to relaxed and linear DNA as determined by electrophoresis, which was similar to the change in DNA due to in vitro treatment with benzo(a) pyrene diol epoxide. Treatment with SMBP completely denatured DNA under alkaline conditions. However, DNA was nicked or partially denatured under neutral condition. The absorption band of DNA was increased by the treatment with SMBP in V79 cells, which may be explained by the formation of stabilized SMBP-DNA adduct.