Template-primers constructed of phiX174 single-stranded viral DNA hybridized to a restriction fragment of phiX174 RF DNA can be used for extensive polymerization by DNA polymerase alpha. Polymerization is dependent upon a restriction fragment containing a 3'OH. The products of the reaction have been identified by agarose gel electrophoresis. Polymerization of 150--400 nucleotides can be obtained in 1h depending upon the restriction fragment used as primer. Synthesis may be limited by barriers in the primary or secondary structure of the template. A factor which stimulates the rate of alpha polymerase activity on these templates was partially purified. This factor does not stimulate alpha polymerase on activated DNA. The stimulating factor sediments at 5.5 S in glycerol gradients containing 0.4M potassium phosphate and has an apparent molecular weight of 70 000 on Sephadex G-100.