We have used the electrophysiological technique of patch-clamp to study the modulation of Escherichia coli porins by cadaverine. Porin channels typically have a very high probability to be open, and were not known to be inhibited by specific compounds until the present study. Experiments performed on patches of outer membrane reconstituted in liposomes reveal that cadaverine applied to the periplasmic side increases the frequency of channel closures in a concentration-dependent fashion, and thereby decreases the total amount of ion flux through a porin-containing membrane. The positive charge on cadaverine is important for inhibition, because the effect is relieved at higher pH where fewer polyamine molecules are charged. Modulation is observed only at negative pipet voltages, and therefore confers voltage dependence to porin activity. Cadaverine increases the number and duration of cooperative closures of more than one channel, suggesting that it does not merely block the pore but exerts its kinetic effect allosterically. As a biological assay of porin inhibition, E. coli behavior in chemotaxis swarm plates was tested and found to be impaired in the presence of cadaverine. Polyamines are naturally found associated with the outer membrane of E.coli, but are lost upon fractionation. We postulate that cadaverine might be a natural regulator of porin activity.