Three Escherichia coli phages, TuIa, TuIb, and TuII, were isolated from local sewage. We present evidence that they use the major outer membrane proteins Ia, Ib, and II, respectively, as receptors. In all cases the proteins, under the experimental conditions used, required lipopolysaccharide to exhibit their receptor activity. For proteins Ia and II, an approximately two- to eightfold molar excess of lipopolysaccharide (based on one diglucosamine unit) was necessary to reach maximal receptor activity. Lipopolysaccharide did not appear to possess phage-binding sites. It seemed that the lipopolysaccharide requirement reflected a protein-lipopolysaccharide interaction in vivo, and lipopolysaccharide may thus cause the specific localization of these proteins. Inactivation of phage TuII by a protein II-lipopolysaccharide complex was reversible as long as the complex was in solution. Precipitation of the complex with Mg2+ led to irreversible phage inactivation with an inactivation constant (37 degrees C)K = 7 X 10-2 ml/min per microgram. With phages TuIa and TuIb and their respective protein-lipopolysaccharide complexes, only irreversible inactivation was found at 37 degrees C. The activity of the three proteins as phage receptors shows that part of them must be located at the cells surface. In addition, the association of proteins Ia and Ib with the murein layer of the cell envelope makes this pair trans-membrane proteins.