Protein E1, a porin of the outer membrane of Pseudomonas aeruginosa, was reconstituted into planar lipid bilayers. Single channel conductance of the protein appeared to be 230 pS (pico siemens) in 1 M KCl-10 mM Hepes, pH7.2. This value is approximately 5 times lower than the conductance of the OmpF channel of Escherichia coli. Conductance increased linearly as the membrane potential was raised from -200 mV to +200 mV, and was nearly proportional to the KCl concentration. These results show that protein E1 is probably a genuine porin in the P. aeruginosa outer membrane supporting the earlier conclusion that protein E1 forms a small channel.