Escherichia coli grown in low‐salt borth (LBS) showed rapid induction of acid sensitivity when L‐leucine was added. Induction was strongly inhibited by nalidixic acid and by tetracycline but only slightly by chyloramphenicol and rifampicin. Regulation of the effect appeared to involve H‐NS, CysB, the ferric uptake regulator (Fur and (p)ppGpp; ompA mutants were also unable to mount the response. Acid sensitization was greatly reduced by glucose, ferric cloride and ferrous sulphage but was not affected appreciably by phosphate or amiloride. Lesions abilishing synthesis of intergration host factor, the PhoE porin and the NhaA antiporter, did not prevent sentiízation by L‐leucine and lrp lesions only slightly affected the response. Organisms exposed to L‐leucine were less susceptibkle to phage K3. Acid sensitivity dependes on appearance of a new outer emebrane (OM) pathways for protons and L‐leucine‐induced organisms may use a new pore formed from a modified OmpA protein.