One- and two-diminsional tryptic and chymotryptic peptide maps of 125-I-labeled alpha and alphabeta avian myeloblastosis virus DNA polymerase demonstrate that the alpha polypeptide of the one and two subunit enzymes are structurally similar, if not identical. Furthermore, the beta subunit contains the same major 125I-labeled peptides as alpha, plus several additional peptides. These relationships and the fact that aging of purified alphabeta avian myeloblastosis virus DNA polymerase increases the proportion of alpha DNA polymerase that can be isolated from the alphabeta enzyme by phosphocellulose chromatography, suggests that alpha is derived from beta by proteolytic cleavage.