The T7 gene 4 protein, a protein known from genetic analysis to participate in phage DNA replication in vivo, has been purified approximately 500-fold with an in vitro complementation assay. The protein, purified from cells infected with a T7 gene 4 temperature-sensitive mutant, is thermolabile, establishing that the complementation activity is in the protein product of the phage gene 4. The purified protein has no detectable nuclease, DNA polymerase, or RNA polymerase activity. However, in addition to stimulating the rate of DNA replication in crude extracts of T7 gene 4 mutant-infected cells, the gene 4 protein effects a marked stimulation of DNA synthesis by the purified T7 DNA polymerase when duplex T7 DNA is used as template. This effect is not observed when denatured T7 DNA is used as template, or when phage T4 DNA polymerase or Escherichia coli DNA polymerase I, II, OR III is substituted for the T4 enzyme. Analysis of the DNA synthesized by the T7 DNA polymerase in the presence of the gene 4 protein indicates that much of the product is in short DNA chains which are not covalently attached to the template. This result suggests a novel mechanism for the initiation of DNA chains in this reaction.