A 75,000-dalton protein complex purified from membranes of competent Bacillus subtilis cells was previously shown to be involved in both binding and entry of donor DNA during transformation. The complex, consisting of two polypeptides, a and b, in approximately equal amounts, showed strong DNA binding as well as nuclease activity (H. Smith, K. Wiersma, S. Bron, and G. Venema, J. Bacteriol. 156:101-108, 1983). In the present experiments, peptide mapping indicated that the two polypeptides are not related. Chromatography on benzoylated, naphthoylated DEAE-cellulose showed that polypeptide b generated single-stranded regions in double-stranded DNA. A considerable amount of the DNA was rendered acid soluble by polypeptide b. The nuclease activity of polypeptide b was reduced in the presence of polypeptide a. This resulted in an increased fraction of high-molecular-weight double-stranded DNA containing single-stranded regions. The acid-soluble DNA degradation products formed by polypeptide b consisted exclusively of oligonucleotides. In contrast to its nuclease activity, which was specifically directed toward double-stranded DNA, the DNA binding of the native 75,000-dalton complex to single-stranded DNA was at least as efficient as to double-stranded DNA.