We previously reported on an SOS-induced toxin, TisB, in Escherichia coli and its regulation by the RNA antitoxin IstR-1. Here, we addressed the mode of action of TisB. By placing the tisB reading frame downstream of a controllable promoter on a plasmid, toxicity could be analysed in the absence of the global SOS response. Upon induction of TisB, cell growth was inhibited and plating efficiency decreased rapidly. The onset of toxicity correlated with a drastic decrease in transcription, translation and replication rates. Cellular RNA was degraded, but in vitro experiments showed that TisB did not affect translation or transcription directly. Thus, these effects are downstream consequences of membrane damage: TisB is predicted to be hydrophobic and membrane spanning, and Western analyses demonstrated that this peptide was strictly localized to the cytoplasmic membrane fraction. Membrane damage and cell killing under tisB multicopy expression are also seen by live/death staining and the formation of ghost cells. This is reminiscent of another toxin, Hok of plasmid R1, which also targets the membrane. The biological significance of the istR/tisB locus is still elusive; deletion of the entire locus gave no fitness phenotype in competition experiments.