A deletion mutation (lpp delta 9 delta 13 delta 14) in the signal peptide of the major outer membrane lipoprotein of Escherichia coli (Lpp) was found to cause severe effects on cell physiology, resulting in cessation of growth within 10 min of induction of lpp delta 9 delta 13 delta 14 expression and rapid cell death. Further investigation revealed that lpp delta 9 delta 13 delta 14 expression caused slow processing of several other exported proteins. The origin of this effect was traced to depolarization of the electrochemical potential across the cytoplasmic membrane, which is known to be required for efficient protein export. Analysis of the processing rate of the mutant, either prior to complete depolarization or in a suppressor strain in which depolarization does not occur, indicates that the mutant protein was capable of secretion at a rate which, while less than that of the wild type, was reasonably rapid compared with the rates of other E. coli secreted proteins. The existence of this type of signal peptide mutation suggests that the cell may have a mechanism to avoid membrane damage from secretory proteins carrying membrane-active signal peptides which is bypassed by the lpp delta 9 delta 13 delta 14 mutant.