Escherichia coli K12 strains producing reduced amounts of LamB protein (one tenth the wild type amount) grow normally on dextrins but transport maltose at a concentration of 1 microM at about one tenth the normal rate. Dex--lamB missense mutants were found as derivatives of these strains. These Dex- mutants had a structurally altered LamB protein in the outer membrane, impaired in its interaction with phages lambda and K10, inefficient in maltose transport at low concentration (10 microM and below). The Dex- mutants still produced one tenth the wild type amount of the LamB protein as the parental Dex+ strains. Analysis of Dex+ revertants showed that the phenotype reverts to Dex+ when the altered LamB protein is made in wild type amounts. Even though they were Dex+, these revertants were still drastically altered in maltose transport at low concentration.