Monoclonal antibodies directed against the alpha subunit of the DNA polymerase III holoenzyme (1) of E. coli were tested for cross-reactivity with a variety of polymerases. We found that one monoclonal antibody bound to E. coli DNA polymerase I as well as to DNA polymerase III. A weaker, but specific, interaction was also detected with T4 DNA polymerase. We exploited the proteolysis procedure developed by Setlow, Brutlag and Kornberg (2) to determine which domain of DNA polymerase I contained the conserved epitope. Contrary to expectations, it was not found in the polymerase domain, but in the 5'----3' exonuclease domain. This reveals a sequence or structure, sufficiently important to be conserved among these polymerases, that is not directly involved in the polymerization reaction.