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Acinetobacter 199A carries on the outer surface of its outer membrane a layer of regularly arranged protein subunits. The isolated surface protein assembles into the same regular array even in the absence of the underlying outer membrane. Cl- minus is required for this self-assembly. Evidence is presented that the interaction of the surface protein with the outer membrane involves the linking of a carboxyl group in the surface protein to a negatively charged group in the outer membrane protein, via a divalent cation. The surface protein could be detached from the outer membrane by the protein perturbant urea, by the chelating agent EDTA and by replacing Mg-2+ with Na+. It could not be detached by treatment with phospholipases A anc D or the detergents Tween 80 and sodium deoxycholate. The conditions favourable for reattachment of surface protein to the cell wall were the presence of divalent cations and a pH of 3-5. Conversion of carboxyl groups in the surface protein to amine with carbodiimide and ethylene diamine interfered with reattachment. The surface protein did not attach to isolated cell wall lipid or lipopolysaccharide.