During the exposure of rat livers to the hepatocarcinogen 2-acetylaminofluorene (N-2-fluorenylacetamide), it was demonstrated that the cytoplasmic DNA polymerase-alpha (E.C. 2.7.7.7) was strikingly error-prone when compared to that of normal liver (Chan, J.Y.H. and Becker, F.F. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 814-818). The fidelity of polymerization of these enzymes was assayed by determining the incorporation of noncomplementary deoxyribonucleotide triphosphates (misincorporation) on a poly(dA-dT) template. To identify the mechanism of infidelity, we modified and extended our purificaton scheme. As a result, a subspecies of polymerase-alpha 1 was identified and separated from the normal component, polymerase-alpha 2. Polymerase-alpha 1 activity eluted from a phosphocellulose column at 0.07-0.12 M NaCl, while polymerase-alpha 2 eluted at 0.15-0.2 M NaCl. Polymerase-alpha 2 demonstrated normal fidelity throughout the various steps of purification while polymerase-alpha 1, despite being purified some 10 250-fold, continued to demonstrate a severe degree of infidelity.