Escherichia coli omega protein is found to form a complex with single-stranded DNA. The complex is stable in buoyant CsCl or Cs2SO4 density gradients. Addition of Mg(II) to the concentrated salt solutions, however, leads to the dissociation of the complex, even in the presence of EDTA in molar excess over Mg(II). The dissociated omega retains its enzymatic activity; the DNA recovered from the dissociated complex is indistinguishable from the original DNA. Exposure of the complex to alkali results in the cleavage of the DNA. This cleavage generates a 3'-hydroxyl DNA terminus, and the omega protein is found linked to the 5'-terminus, presumably covalently. Pronase digestion of the complex results initially in the removal of approximately 30% of the protein. A significant fraction of the residual complex is still stable in concentrated salt solutions, and can be dissociated by Mg(II). Extensive digestion with pronase results in the removal of the protein and the cleavage of the DNA chain.