DNA polymerase III (delta) of Saccharomyces cerevisiae is purified as a complex of at least two polypeptides with molecular masses of 125 and 55 kDa as judged by SDS-PAGE. In this paper we determine partial amino acid sequences of the 125 and 55 kDa polypeptides and find that they match parts of the amino acid sequences predicted from the nucleotide sequence of the CDC2 and HYS2 genes respectively. We also show by Western blotting that Hys2 protein co-purifies with DNA polymerase III activity as well as Cdc2 polypeptide. The complex form of DNA polymerase III activity could not be detected in thermosensitive hys2 mutant cell extracts, although another form of DNA polymerase III was found. This form of DNA polymerase III, which could also be detected in wild-type extracts, was not associated with Hys2 protein and was not stimulated by addition of proliferating cell nuclear antigen (PCNA), replication factor A (RF-A) or replication factor C (RF-C). The temperature-sensitive growth phenotype of hys2-1 and hys2-2 mutations could be suppressed by the CDC2 gene on a multicopy plasmid. These data suggest that the 55 kDa polypeptide encoded by the HYS2 gene is one of the subunits of DNA polymerase III complex in S.cerevisiae and is required for highly processive DNA synthesis catalyzed by DNA polymerase III in the presence of PCNA, RF-A and RF-C.