Lipid-bilayer experiments were performed with the related hemolysins from Proteus vulgaris and Morganella morganii (HlyA). The addition of the toxins to the aqueous phase bathing lipid-bilayer membranes composed of different lipids resulted in the formation of transient ion-permeable channels. Membranes formed of pure lipids were rather inactive targets for the hemolysins as compared with lipid mixtures such as asolectin. The channels had several different substrates. The major open state had single-channel conductances of 500 pS in 0.15 M KCl at small transmembrane voltages. Experiments with different salts suggested that the hemolysin-induced channels of P. vulgaris and M. morganii were exclusively cation selective at neutral pH, caused by negative charges localized at the channel mouth. The mobility sequence of the cations within the channels was similar if not identical to their mobility sequence in the aqueous phase. The single-channel data were consistent with wide, water-filled channels with estimated minimal diameters of about 1 nm since the large organic cation Tris+ can permeate the channels without any detectable interaction with its interior. Pore-forming properties of these hemolysins were compared with those of HlyA of Escherichia coli. All these toxins share common features, oligomerize probably to form pores in lipid-bilayer membranes and form channels with similar properties which suggests that their structures are more or less identical.