An extracellular induction component (EIC), needed for acid tolerance induction at pH 5.0 in Escherichia coli, arises from an extracellular precursor which senses acid stress and is activated (forming the EIC) by such stress. The precursor, which is a heat-stable protein, was formed by cells which had not been subjected to acid stress, being present in culture media after growth at pH values from 7.0 to 9.0. This stress-sensing molecule was activated to the EIC at pH values from 4.5 to 6.0 but not at pH 6.5 and did not form EIC on incubation at an extremely acidic pH e.g. 2.0. The precursor was not inactivated at pH 2.0. Precursor activation might be reversible, as the EIC lost its ability to induce acid tolerance after incubation at pH 9.0, but regained it if subsequently incubated at pH 5.0. Whereas the sensor formed at pH 7.0 can only be activated at pH 5.0 to 6.0, that synthesized at pH 9.0 can be activated at pH 5.0 to 7.5. Accordingly, this work shows that the acid stress sensor is extracellular, and it is proposed that its presence in the medium rather than in the cells, allows more sensitive and rapid responses to acid stress.