The in vitro activity of many pore-forming toxins, in particular, the rate of increase in the membrane conductance induced by the channel-forming domain (P178) of colicin E1 is maximum at an acidic pH. However, after P178 binding at acidic conditions, a subsequent pH shift from 4 to 6 on both sides of the planar bilayer lipid membrane caused a large increase in the trans-membrane current which was solely due to an increase in the number of open channels. This effect required the presence of anionic lipid. Replacing the His440 residue of P178 by alanine eliminated the pH-shift effect thereby showing that it is associated with deprotonation of this histidine residue. It was concluded that alkalinization-induced weakening of the electrostatic interactions between colicin and the membrane surface facilitates conformational changes required for the transition of membrane-bound colicin molecules to an active channel state.