Consistent with previous observations, proliferating cell nuclear antigen (PCNA) promotes DNA synthesis by calf thymus DNA polymerase delta (pol delta) past several chemically defined template lesions including model abasic sites, 8-oxo-deoxyguanosine (dG) and aminofluorene-dG (but not acetylaminofluorene-dG). This synthesis is potentially mutagenic. The model abasic site was studied most extensively. When all deoxyribonucleoside triphosphates and a template bearing a model abasic site were present, DNA synthesis by pol delta beyond this site was stimulated 53-fold by addition of homologous PCNA. On an unmodified template (lacking any lesions), PCNA stimulated pol delta by 1.3-fold. Product analysis demonstrated that as expected from the "A-rule," fully and near-fully extended primers incorporated predominantly dAMP opposite the template lesion. Moreover, corollary primer extension studies demonstrated that in the presence (but not the absence) of PCNA, pol delta preferentially elongated primers containing dAMP opposite the model abasic template site. p21, a specific inhibitor of PCNA-dependent DNA replication, inhibits PCNA-stimulated synthesis past model abasic template sites. We propose that DNA synthesis past template lesions by pol delta promoted by PCNA results from the fundamental mechanism by which PCNA stimulates pol delta, i.e., stabilization of the pol delta. template-primer complex.