Primer recognition proteins (PRP) are cofactors of DNA polymerase alpha and may have a role in lagging strand DNA replication. Purified PRP from HeLa cells and human placenta are composed of two subunits of 36,000 (PRP 1) and 41,000 (PRP 2) daltons. Upon tryptic digestion, amino acid sequencing of tryptic peptides, and homology search against a protein sequence data base, we have identified PRP 2 to be the glycolytic enzyme, phosphoglycerate kinase (PGK). The activities of PRP and PGK increase coordinately in the PRP purification procedure. PRP activity is inhibited by the PGK substrate 3-phosphoglycerate and the competitive inhibitor of substrate binding, DL-alpha-glycerol 3-phosphate. 5'-p-Fluorosulfonylbenzoyl adenosine, which inactivates PGK by binding to the nucleotide binding site, also inhibits PRP. For PRP activity, the two substrate binding sites of PGK are necessary in addition to the as yet unidentified PRP 1 polypeptide.