Mouse myeloma DNA polymerase gamma was extensively purified to a final specific activity of 156 000 units (nmol dTMP incorporation per h) per mg protein on (rA)n.(dT)12-18 as a template primer. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of protein fractions obtained by DNA-cellulose column chromatography revealed that the amount of a polypeptide of Mr = 47 000 changed proportionally with DNA polymerase gamma activity. A minor polypeptide of Mr = 140 000 also seemed to change with the enzyme activity, but other polypeptides did not. Analysis by 125I-labeled peptide mapping indicates that the Mr 47 000 polypeptide in the mouse myeloma DNA polymerase gamma preparation is structurally related to the Mr 47 000 polypeptide of chick embryo DNA polymerase gamma (Yamaguchi, M., Matsukage, A. and Takahashi, T. (1980) J. Biol. Chem. 255, 7002-7009). An antibody against chick embryo DNA polymerase gamma cross-reacted with the mouse enzyme, indicating a structural relationship between avian and murine enzymes. Since the Mr 47 000 polypeptide accounts for 31.4% of total protein in the purified preparation, the specific activity is estimated to be about 490 000 units per mg of the Mr 47 000 polypeptide. The rate of poly(dT) elongation by the purified enzyme was 1 260 per nucleotides per min. This value is in the same range as the turnover number (1 530 nucleotides per min per enzyme molecule) which is calculated from the 'expected' specific activity with respect to the Mr 47 000 polypeptide and the molecular weight (Mr = 188 000 on the assumption of a tetrameric structure of the Mr 47 000 polypeptide). Results indicate that the Mr 47 000 polypeptide is a component of the mouse myeloma DNA polymerase gamma.