Steady-state kinetic studies indicated that the alpha subunit incorporated 2-OH-dATP 10 times more frequently opposite T than opposite G. On the other hand, the incorporation of 2-OH-dATP opposite T by the exonuclease-deficient Klenow fragment was 2 orders of magnitude more efficient than that opposite G. These results indicate that the misinsertion specificity of 2-OH-dATP differs between replicative and repair-type DNA polymerases, and provide a biochemical basis for the mutations induced by 2-OH-dATP in E. coli.