The T4 bacteriophage gene 59 protein is required for normal T4 DNA replication. We have purified this protein to homogeneity in two steps and show that it binds both to single-stranded DNA and to the T4 gene 32 protein, a DNA single strand binding protein. In in vitro assays, covering DNA with 32 protein makes this DNA inaccessible to the 41 protein, the highly processive DNA helicase, that associates with the T4 DNA primase (gene 61 protein) to form an active primosome. However, the 59 protein brings about the rapid assembly of 41 protein onto single-stranded DNA, even if this DNA is covered with 32 protein. The 59 protein is therefore a DNA helicase assembly protein. The observed requirements for the 59 protein in the vivo T4 DNA replication are explained by there being two alternative pathways for loading the 41 protein onto a replication fork at early times of T4 DNA synthesis, with only a 59 protein-mediated pathway remaining operative for the recombination-mediated replication that dominates later in infection (Barry, J., and Alberts, B. M. (1994) J. Biol. Chem. 269, 33063-33068).