Escherichia coli DNA polymerase I shows a delay in the initiation of polymerization after binding to the cohesive ends of lambda DNA. This delay is significantly longer than the time required for the synthesis of an octanucleotide sequence on the right-hand cohesive end of the lambda DNA. When the extent of polymerization is limited by omission of one or more of the deoxynucleoside triphosphates, and polymerization started again by their addition, the delay still occurs. A plausible explanation for this phenomenon is that two forms of the enzyme or enzyme - DNA complex exist, only one of which is active. The delay, therefore, represents the time necessary to convert the inactive to the active form of the enzyme or enzyme complex. One consequence of the defect in DNA polymerase I, due to the polA12 mutation, is apparently to alter the equilibrium between the two forms. However, the rate of polymerization and the rate of conversion of inactive to active enzyme or enzyme complex are not changed significantly.