Escherichia coli mutants that are insensitive to colicins B and I hyperproduce and excrete the iron chelator enterochelin, which is an inhibitor of these colicins. These mutants are classified as exbA and exbB. The exbA mutants are chromium sensitive and require iron for growth, and the mutations are located in the tonB region at min 25 of the E. coli chromosome. tonB mutants in which the genome of phage lambda is inserted into the bacterial chromosome within the tonB gene also exhibit enterochelin excretion. The exbB mutants require methionine and probably result from deletions which are located between min 56 and 58. Colicin insensitivity, enterochelin excretion and methionine auxotrophy are recessive in exbB merodiploids. The methionine requirement of exbB strains is satisfied by cystathionine or homocysteine, and exbB mutants are sensitive to ethionine.