A 107 kDa hemolysin from Escherichia coli is able to open pores in lipid membranes. By studying its interaction with planar phospholipid bilayers we have derived some structural information on the organization of the pore. We measured the current-voltage characteristic and the ion selectivity of the channel both in neutral membranes, made of egg phosphatidylcholine (PC) and in negatively charged membranes, made of a 1:1 mixture of PC with phosphatidylserine (PS). Experiments were performed varying both the pH and the salt concentration of the bathing KCl solution. In neutral membranes the pore is ohmic and its conductance increases almost linearly with the salt concentration. The channel is cation-selective at high pH but nearly unselective at low pH. We interpret these results in terms of a minimal model based on classical electro-diffusional theories assuming that the pore is wide and bears a negative charge at its entrances. In membranes containing the acidic lipid the current-voltage curve is non-linear in such a way to suggest that the trans (but not the cis) entrance of the pore is affected by the surface potential of the membrane. Applying our model we find that the trans and cis entrances are located, respectively, about 0.5 nm and more than 5 nm apart from the plane of the membrane. We confirmed the asymmetric disposition of the channel by enzymatic digestion of preformed pores. This was effective only when the enzyme was applied on the cis side.