Escherichia coli strains which carry mutations in the to1F or to1G loci have alterations in the protein composition of their outer membranes. A mutation at either loci results in the loss of a different major outer membrane protein. These mutants are tolerant to colicin L-JF246 and resistant to at least one bacteriophage. The goal of the research proposed is to study the synthesis, assembly and biological role of the proteins missing in to1F and to1G mutants. We have purified protein preparations which we have characterized chemically and hope to develop a biological assay using the purified material to reconstitute bacteriocin and/or bacteriophage sensitivity in tolerant mutants.