Escherichia coli K-12 strains carrying mutations in the ompB gene or double mutations in the tolF and par genes lack the major outer membrane proteins 1a and 1b. These strains are deficient in the transport of small hydrophylic compounds and are multiply colicin resistant. When revertants of these strains were sought, a number of extragenic pseudorevertants were obtained which produced new outer membrane proteins. These new proteins could be divided into three classes by differences in electrophoretic mobility on polyacrylamide gels, by differing specificities for transport of small molecules, and by the identification of three different genetic loci for genes controlling their production. These genetic loci are designated as nmpA (at approximately 82.5 min on the E. coli K-12 genetic map), nmpB (8.6 min), and nmpC (12 min). The new proteins produced in strains carrying nmpA, nmpB, or nmpC mutations did not cross-react with antiserum against a mixture of proteins 1a and 1b, or with antiserum against phage-directed protein 2. Production of the new membrane proteins restored sensitivity to some of the colicins.