The contribution of human DNA polymerase epsilon to nuclear DNA replication was studied. Antibody K18 that specifically inhibits DNA polymerase activity of human DNA polymerase epsilon in vitro significantly inhibits DNA synthesis both when microinjected into nuclei of exponentially growing human fibroblasts and in isolated HeLa cell nuclei. The capability of this neutralizing antibody to inhibit DNA synthesis in cells is comparable to that of monoclonal antibody SJK-132-20 against DNA polymerase alpha. Contrary to the antibody against DNA polymerase alpha, antibody K18 against DNA polymerase epsilon did not inhibit SV40 DNA replication in vitro. These results indicate that DNA polymerase epsilon plays a role in replicative DNA synthesis in proliferating human cells like DNA polymerase alpha, and that this role for DNA polymerase epsilon cannot be modeled by SV40 DNA replication.