The role of the KefB and KefC potassium efflux systems in protecting Escherichia coli cells against the toxic effects of the electrophile N-ethylmaleimide has been investigated. Activation of KefB and KefC aids the survival of cells exposed to high concentrations (> 100 microM) of NEM. High potassium concentrations reduce the protection afforded by activation of KefB and KefC, but the possession of these systems is still important under these conditions. The Kdp system, which confers sensitivity to the electrophile methylglyoxal, did not affect the survival of cells exposed to NEM. Survival is correlated with the reduction of the cytoplasmic pH upon activation of the channels. In particular, the kinetics of the intracellular pH (pHi) change are crucial to the retention of viability of cells exposed to NEM; slow acidification does not protect cells as effectively as rapid lowering of pHi. Cells treated with low levels of NEM (10 microM) recover faster if they activate KefB and KefC, and this correlates with changes in pHi. The pHi does not significantly alter the rate of NEM metabolism. The possible mechanisms by which protection against the electrophile is mediated are discussed.