The catalytic polypeptide of DNA polymerase α is often observed in vitro as a family of phosphopolypeptides predominantly of 180 and 165 kDa derived from a single primary structure. The estimated Mr of this polypeptide deduced from the full-length cDNA is 165 kDa. Immunoblot analysis with polyclonal antibodies against peptldes of the N- and C-terminl of the deduced primary sequence indicates that the observed family of polypetldes from 180 kDa to lower molecular weight results from proteolytic cleavage from the N-termlnus. Antibodies against the N-terminal peptide detect only the 180 kDa species suggesting that this higher molecular weight polypeptide may be the result of posttranslational modification of the 165 kDa primary translation product. The catalytic polypeptide is not only phosphorylated but is also found to react with lectins ConA and RCA. N-terminal sequencing of the isolated catalytic polypeptide from human cells and of the recombinant fusion proteins indicates that the often observed 165 kDa polypeptide is the in vitro proteolytic cleavage product of the modified 180 kDa protein at the specific site between lys 123 and lys 124 within the sequence -RNVKKLAVTKPNN-.