We have previously shown that the processive synthesis of long DNA products on a poly(dA) X oligo(dT)10 primer-template is facilitated by formation of an isolable initiation complex between the Escherichia coli DNA polymerase III holoenzyme and DNA in the presence of ATP (Fay, P. J., Johanson, K. O., McHenry, C. S., and Bambara, R. A. (1982) J. Biol. Chem. 257, 5692-5699). Here we have demonstrated that the ATP requirement for processive synthesis can be obviated by a large excess of the beta subunit of the DNA polymerase III holoenzyme. The reaction which occurs in the presence of excess beta can be distinguished from the ATP-mediated reaction by its salt sensitivity and the lack of stabile initiation complex formation between polymerase and primed DNA. A model is presented which suggest that one of the functions of ATP in the DNA polymerase III holoenzyme reaction is to lock beta into the replicative complex such that it does not readily equilibrate with solution.