A simple kinetic analysis of the values of kcat and KM for base insertion and misinsertion during DNA replication is presented and applied to the problem of base misinsertion by DNA polymerase I of Escherichia coli. The role of minor tautomeric forms of deoxynucleoside triphosphates (dNTPs) in purine x pyrimidine mismatching has been examined and it has been shown that the misinsertion frequency via this route should be close to the tautomerization constant in solution and is independent of any effect of the polymerase on the tautomerization of a dNTP when bound. Kinetic data on purine x pyrimidine mismatching indicate that the dNTP in a polymerase-DNA-mismatched-dNTP complex is predominantly in the major tautomeric form. The mutagenic effect of Mn2+ in DNA replication is shown to be mediated by decreasing the values of kcat/KM for the insertion of correct dNTPs, whilst the values of this rate constant for misinsertion are relatively unaffected or increased.