A DNA-binding protein has been purified from Escherichia coli infected with bacteriophage T7 by DNA-cellulose chromatography. The protein is absent in uninfected cells. The purified protein has a molecular weight of 31,000 and binds strongly and preferentially to single-stranded DNA. In vitro studies show that this protein can stimulate the rate of polymerization catalyzed by the T7-induced DNA polymerase 10-15 times under conditions where the polymerase is unable to effectively use a single-stranded template. The degree of stimulation is dependent upon the ratio of binding protein to DNA template and is independent of polymerase concentration. The observed stimulation is specific for the T7 DNA polymerase in that addition of the protein to reactions catalyzed by E. coli DNA polymerases I, II, or III or T4 DNA polymerase is without effect.