Gene 59 of bacteriophage T4 is involved in genetic recombination and recombination-dependent DNA replication. We purified the protein encoded by gene 59 after cloning the gene in an expression vector. The molecular weight and the sequence of seven N-terminal amino acids of the protein were consistent with the nucleotide sequence of the gene (Hahn, S., Kruse, U., and Ruger, W. (1986) Nucleic Acids Res. 14, 9311-9327). The purified 59 protein appeared to be a monomer under nondenaturing conditions, and it bound to single-stranded DNA in preference to double-stranded DNA. The protein could bind to a 24-nucleotide-long single-stranded DNA molecule that had previously bound either 32 protein (single-stranded DNA-binding protein) or uvsY protein (enhancer of uvsX protein), but it would not bind to this DNA if it had previously bound uvsX protein (synaptase). The binding occurred rapidly with 32 protein-bound DNA and slowly with uvsY protein-bound DNA. Accordingly, 32 protein bound specifically to an agarose matrix containing immobilized 59 protein. The 41 protein (DNA helicase) also bound specifically to this agarose matrix.