A protein factor which stimulates DNA polymerase α activity on heat-denatured DNA has been purified from mouse FM3A cells. The final preparation had a specific activity of 43, 000 units/mg protein and lacked detectable DNA polymerase, RNA polymerase, DNA-dependent- and independent ATPase, exo- and endodeox-yribonuclease and phosphatase activities. The stimulating factor sedimented at 2.9 S in a glycerol gradient. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of the glycerol gradient fraction revealed the presence of a major band of 36, 000 daltons, the amount of which corresponded well with the level of stimulating activity. The stimulation by the factor was specific for heat-denatured DNA, and a little or no stimulation was observed with native DNA, ribo- and deoxyribohomo-polymers and single stranded circular DNA. Alkaline sucrose gradient sedimentation analysis of the reaction products revealed that newly synthesized DNA was covalently linked to the termini of heat-denatured DNA. The average chain length of the elongated span determined by the digestion with micrococcal nuclease and phosphodiesterase II, did not differ between in the presence and absence of the stimulating factor, suggesting that the stimulation by the factor was due to the increase in the initiation frequency of DNA synthesis from the 3'-hydroxyl terminus of heat-denatured DNA.