The effect of protamine on growing cells of Escherichia coli K-12 strains containing different K+-uptake systems was investigated. Immediately after the addition of the toxic peptide, growth ceased and all strains lost most of their K+. In addition, these cells released a significant amount of their ATP into the medium, and the cytoplasmic volume of these cells decreased by 70%. Whereas cells without rapid K+-uptake systems did not recover, cells containing either the Trk systems or the overproduced Kup system slowly reversed the effects of protamine, and growth resumed after the cells had reached their original volume. Experiments with a set of strains carrying mutations in the K+-uptake gene trkA showed a reasonably satisfactory correlation between inhibition of net K+ uptake and the lag time for resumption of growth after addition of protamine. Cells carrying mutations in three extracytoplasmic proteases were hypersusceptible to protamine, suggesting that the toxic peptide is degraded by these proteases. Data on the effect of a second addition of protamine suggest that protamine degradation activity is inducible. These data are interpreted to mean that reaccumulation of K+ by protamine-treated cells triggers recovery of the cells, thereby allowing induction of extracytoplasmic proteases. These, in turn, degrade protamine, leading to complete recovery of the cells and resumption of growth. Cells that cannot take up K+ rapidly remain metabolically compromised to such an extent that extracytoplasmic protease activity is not induced, leading to a prolonged susceptibility of the cells to the toxic peptide.