Porin is a channel-forming protein in the outer membrane of Gram-negative bacteria. In the previous paper (Todt et al., 1992), we showed that the pH induced a switch in the channel size in vitro for the porins OmpF, OmpC, and PhoE. In the results presented here, His21 of OmpC and OmpF from Escherichia coli was chemically modified with diethyl pyrocarbonate. Functional analysis of these modified porins at different pHs suggested that this histidine is involved in the pH-induced switch in channel size. Secondary structure analysis of porins at various pHs using Fourier transform infrared spectroscopy indicated that there was no global change in structure accompanying the pH-induced switch in channel size.