We have used antibodies specific for either polymerase delta (pol delta) or its accessory protein, proliferating cell nuclear antigen (PCNA), to demonstrate that they can markedly inhibit the capacity of HeLa nuclear extracts to effect repair of UV-damaged plasmid DNA. This provides the first unambiguous evidence for the involvement of pol delta in DNA repair synthesis. The mRNA levels of both pol delta and PCNA were significantly stimulated subsequent to UV irradiation of cultured cells, providing the first evidence that the cellular response to UV damage may involve regulation of pol delta and PCNA at the gene level. Thus, DNA polymerase delta and its accessory proteins, in addition to their function in replicative DNA synthesis, also function in DNA repair synthesis.