The DNA polymerase induced by an antimutator T4 phage has been purified to apparent homogeneity and has been compared to the wild type polymerase. The mutant enzyme resembles the wild type in thermal stability, pH optimum, salt activation, divalent metal ion requirement, inhibition by a sulfhydryl reagent, and apparent affinity for DNA. However, the mutant enzyme differs from the wild type in its 8-fold higher 3'-exonuclease activity and in its decreased apparent affinity for deoxyribonucleoside triphosphates. Inhibition studies indicate that the exonuclease of the mutant enzyme is more vulnerable to physical and chemical modification than its wild type counterpart.