Lipid bilayer experiments were performed with the plasmid-encoded hemolysin of enterohemorrhagic Escherichia coli (EHEC) O157:H7 strain EDL933. EHEC-hemolysin caused the formation of transient ion-permeable channels by integration in lipid bilayer membranes composed of asolectin, dioleoylglycerophosphoethanolamine and phosphoserine but not of diphytanoylglycerophosphocholine. Channel formation showed the same characteristics when culture supernatants of E. coli strains EDL 933 or HB101/pEO40, precipitated or purified EHEC-hemolysin were used for these experiments. The EHEC-hemolysin channels had two different states at small transmembrane potential (20 mV): a prestate that represented the first step of channel formation (single-channel conductance 40 pS in 0.15 M KCl) and an open state (550 pS in 0.15 M KCl at pH 6.0). Experiments with different salts suggested that the EHEC-hemolysin-induced channels were cation-selective at neutral pH. The mobility sequence of the cations within the channels resembles their mobility sequence in the aqueous phase. The single-channel data were consistent with the formation of wide, water-filled channels by the EHEC hemolysin. The single channel conductance was strongly pH dependent and increased over 2.5-fold in the pH range 5-8. The analysis of the single-channel data using the Renkin correction factor suggested that the EHEC-hemolysin formed channels with an average diameter of 2.6 nm. This size could be confirmed by the results of osmotic-protection experiments. Neither sucrose nor raffinose inhibited toxin-dependent hemolysis, whereas hemolysis did not occur in the presence of dextran 4 (molecular mass, 4 kDa). Our results demonstrate that EHEC-hemolysin can be considered to be a highly active repeats-in-toxin (RTX)-toxin with a similar but not identical pore-forming capacity as the chromosomal encoded E. coli alpha-hemolysin.