Organisms of Escherichia coli 1829 become alkali sensitized on transfer from pH 7.0 to pH 5.5 but they also secrete extracellular agents which induce alkali sensitivity when added (in neutralized filtrates) to organisms growing at pH 7.0. In contrast, filtrates from cultures grown at pH 7.0 have no effect. Filtrates were inactivated by protease but not by heat treatment in a boiling water-bath, suggesting that a very heat-stable protein is involved in alkali sensitivity induction. A heat-stable low molecular weight component (or components) may also be needed for induction, or the induction protein itself may be of low molecular weight. Strains with lesions in hns, fur or himA produced almost inactive filtrates and it therefore appears that H-NS, Fur and IHF are involved in synthesis of the induction components. As the presence of protease during incubation at pH 5.5 totally abolished alkali sensitization of strain 1829 while inhibition of sensitization induction occurred if the induction components were removed by filtration or dialysis during pH 5.5 incubation, it is proposed that the extracellular induction components (EICs) are essential for the original sensitization response. These results suggest that sensitization induction occurs by a different mechanism to that which is believed to occur for most bacterial inducible response systems; these are claimed to involve exclusively intracellular reactions and components whereas the present response involves functioning of extracellular components.