A DNA polymerase with a 3'-to 5'-exonuclease that copurified with polymerase-primase from calf thymus was purified and extensively characterized. Its exonuclease degraded single-stranded DNA from 3' to 5' in a strictly distributive manner. On synthetic template-primer junctions, 3'-terminal mispairs were excised with a 10- to 20-fold preference over correctly paired nucleotides. In comparison to the 3'- to 5'-exonuclease the DNA polymerase activity was rather low. The ratio of nucleotides incorporated to nucleotides excised was in the order of 1 to 3 nucleotide insertions per excision, suggesting that net forward DNA synthesis is not the primary role of this DNA polymerase. DNA synthesis was performed with a low processivity in the presence and absence of PCNA. Both the polymerase and exonuclease activities were inhibited to a comparable extent by AMP. Thus, the exonuclease-polymerase might represent a novel DNA polymerase that we tentatively designate as DNA polymerase zeta. Possible benefits of DNA polymerase zeta in the process of error correction and the apparent dichotomy of an built-in proofreading activity for the processive DNA polymerases gamma, delta, and epsilon and an obviously external proofreading function for the less processive animal cell DNA polymerases alpha and beta are discussed.