We have performed electrical measurements on bilayer lipid membranes (BLMs) in the presence of nisin. The membranes were formed in 0.1 M KCI solution following the technique of Montal-Mueller as well as that of Rudin-Mueller. Nisin was dissolved in water and introduced in small amount into one side (cis-side) of the solution bathing the membrane to final concentrations of 10(-7)-10(-5) M. By applying external voltages of different polarity, membrane conductance increases were observed. In Montal-Mueller membranes, channels with elementary conductance of about 145 pS were detected both for trans-negative and trans-positive potentials (50 mV). Channels with elementary conductance around 400 pS were observed only for trans-negative potentials. The log-log plot of macroscopic conductance versus nisin concentration was linear with a slope of (1.0 +/- 0.1), showing that one nisin molecule is involved in channel formation.