Effects of polydeoxynucleotides on the activity of DNA polymerase-α from sea urchin embryos were studied. Poly (dG), poly (dC) and poly (dC)-oligo (dG) _12-18 inhibited DNA polymerase-a activity in the activated DNA-directed reaction but poly (dA), oligo (dT) _12-18, and poly (dA)-oligo (dT)_12-18 did not inhibit the activity. The inhibitory mode of poly (dC)-oligo (dG)_12-18 or poly (dC) was competitive with activated DNA and that of poly (dG) was noncompetitive with activated DNA. Using poly (dA)-oligo (dT)_12-18 as a template-primer, the inhibition with either poly(dG) or poly (dC)-oligo (dG)_12-18 was competitive with the template-primer. These kinetic results indicate that each of the template-primers tested binds to an identical site on DNA polymerase-α. Similar results were obtained with DNA polymerase-α from HeLa cells.