Escherichia coli became more acid tolerant following incubation for 60 min in a medium containing l-glutamate at pH 7·0, 7·5 or 8·5. Several agents, including cAMP, NaCl, sucrose, SDS and DOC, prevented tolerance appearing if present with l-glutamate. Lesions in cysB, hns, fur, himA and relA, which frequently affect pH responses, failed to prevent l-glutamate-induced acid tolerance but a lesion in l-glutamate decarboxylase abolished the response. Induction of acid tolerance by l-glutamate was associated with the accumulation in the growth medium of a protein (or proteins) which was able to convert pH 7·0-grown cultures to acid tolerance, and the original l-glutamate-induced tolerance response was dependent on this component(s). Acid tolerance was also induced by l-aspartate at pH 7·0 and induction of such tolerance was dependent on an extracellular protein (or proteins). The l-glutamate and l-aspartate acid tolerance induction processes are further examples of a number of stress tolerance responses which differ from most inductions in that extracellular components, including extracellular sensors, are required.