We have previously reported that sulfoquinovosylmonoacylglycerol (SQMG) is a potent inhibitor of mammalian DNA polymerases. DNA polymeraseb(pol b) is one of the most important enzymes protecting the cell against DNA damage by base excision repair. In this study, we charac-terized the inhibitory action of SQMG against rat pol b. SQMG competed with both the substrate and the template-primer for binding to polb.