Kinetic analysis of DNA polymerase epsilon in its interaction with the homopolymeric template-primer poly(dA)/oligo(dT) and a singly-primed synthetic oligonucleotide of defined sequence indicated that primer utilization is inhibited by single-stranded DNA. Long single-stranded DNA regions appear to sequester DNA polymerase epsilon via nonproductive binding, thus reducing its catalytic efficiency. Proliferating cell nuclear antigen can reduce this nonproductive effect by increasing the rate of primer binding by DNA polymerase epsilon. Once the complex between DNA polymerase epsilon and the primer is formed, proliferating cell nuclear antigen can increase the rate of nucleotide incorporation. The results suggested a dual role of proliferating cell nuclear antigen in stimulating the activity of DNA polymerase epsilon, namely, first to facilitate primer binding and second to stimulate the synthetic activity itself. A model for the interaction between these two proteins in DNA synthesis is discussed.