The homogeneous DNA polymerase alpha from early embryos of Drosophila melanogaster contains four polypeptides designated alpha, beta, gamma, and delta, with molecular weights of 148,000, 58,000, 46,000, and 43,000, respectively (Banks, G. R., Boezi, J. A., and Lehman, I. R. (1979) J. Biol. Chem. 254, 9886-9892). The four polypeptides are structurally distinct from one another, as indicated by their different peptide patterns following limited proteolysis with Staphylococcus aureus protease. Furthermore, the inclusion of the protease inhibitors, leupeptin and pepstatin, in addition to phenpylmethylsulfonyl fluoride and sodium metabisulfite, which are used routinely during the purification, does not alter the pattern of polypeptides in the purified polymerase, suggesting that the four polypeptides are not a consequence of nonspecific proteolysis during purification. Thus, the alpha, beta, gamma, and delta polypeptides appear to be distinct subunits of the alpha-DNA polymerase of D. melanogaster. The alpha subunit is required for DNA polymerase activity. However, the specific activity of the isolated subunit is substantially lower than when it is associated with the beta, gamma, and delta subunits.