Activities of DNA-dependent DNA and RNA polymerases were changed, when the primer DNA had been pre-treated with ascorbic acid, 5-methyl-3,4-dihydroxytetrone, scorbamic acid or 5-mercapto-4-hydroxycoumarin. The activity increased or decreased depending on the reaction condition, such as coexistence of Cu^<2+>, concentration of the reductones or reaction time. The study suggested that the degree of the variation seemed to relate with the breakage of the primer DNA due to the reductone structure.