Leucocidins and y-hemolysins are bi-component staphylococcal toxins that form lytic transmem-brane pores. Their cytotoxic activities involve the synergistic association of a class S and a class F component, produced as water-soluble monomers which assemble on the surface of specific cells. The structure of the F protein from Panton-Valentine leucocidin, solved at 2.0 A resolution, and sequence alignment suggest that it represents the fold of any secreted protein in this family of toxins. The comparison of this structure to that of the homoheptameric a-hemolysin provides some insights into the molecular events that may occur during pore formation.