We have purified a DNA-binding protein from bacteriophage T5-infected cells. The protein is the product of the T5 gene D5 and is produced in quantities ultimately exceeding 2% of the total cell protein. The protein has no tendency to aggregate and exists in solution as a monomer of about 29,000 daltons. Although the protein binds with a high affinity to both single-stranded (fd, T7) and duplex DNA (T5, T7), there is a preferred binding to duplex DNA. The binding of the gene D5 protein to duplex DNA is cooperative with an estimated dissociation constant of 6.27 x 10(-10) M and leads to the production of a more condensed form of the DNA. In contrast, binding of the gene D5 protein to single-stranded DNA is noncooperative with an estimated dissociation constant of only 1.85 x 10(-8) M and produces a sedimentation shift in the DNA proportional to the added mass of the protein. The protein product of the gene D5 of bacteriophage T5 is known to be required for both T5 DNA replication and for the inhibition of early phage gene expression. We also show here that the protein is required to initiate transcription of the late region of the T5 genome.