Isolated nuclei contained two active forms of DNA polymerase alpha (form I and form II). Form II was extracted from nuclei by KCl at concentrations lower than 0.18 M. Above 0.18 M selective extraction of form I was observed. The purified two active forms differed in chromatographic and electrophoretic behaviors, in their salt requirement for optimal activity, and in preference of template-primers, although both forms exhibited properties characteristic of DNA polymerase alpha such as sensitivity of N-ethylmaleimide, 1-beta-D-arabino-furanosylcytosine triphosphate, and aphidicolin. Marked difference between the two forms was preference of template-primer that form I was more active with poly(dT).(rA)10 than poly(dA).(dT)12 whereas form II exhibited higher activity with poly(dA).(dT)12 than poly(dT).(rA)10. Possible roles of two forms of DNA polymerase alpha in the processes of DNA replication will be discussed.