Bacteriophage T7 gene 5 protein has been purified to apparent homogeneity from cells overexpressing its gene several hundred-fold. Gene 5 protein is a DNA polymerase with low processivity; it dissociates from the primer-template after catalyzing the incorporation of 1-50 nucleotides, depending on the salt concentration. Escherichia coli thioredoxin, a host protein that is tightly associated with the gene 5 protein in phage-infected cells, is not required for this activity. Thioredoxin acts as an accessory protein to bestow processivity on the polymerizing reaction; DNA synthesis catalyzed by the gene 5 protein-thioredoxin complex on a single-stranded DNA template can polymerize thousands of nucleotides without dissociation. Conditions that increase the stability of secondary structures in the template (i.e., low temperature or high ionic strength) decrease the processivity. E. coli single-stranded DNA-binding protein stimulates both the rate of elongation and the processivity of the gene 5 protein-thioredoxin complex.