Staphylococcus aureus alpha-toxin was characterized with respect to surface activity and its interaction with lipid monolayers. The protein alone had a detergent-like behavior at the air/water interface. Its affinity was higher for negatively charged than for neutral phospholipids. The interaction was pH dependent, showing a maximum increase at pH 7.0. Only a small part of the protein oligomer appeared to be inserted into the monolayers., crystalline sheets of alpha-toxin were formed using negatively charged phospholipids. Electron microscopy of such areas, at different tilt angles, allowed reconstruction of a three-dimensional model following image processing, The sheets analyzed consisted of two protein layers arranged on a tetragonal lattice. Under the conditions used to grow the crystals the toxin formed 90-Angstrom-wide cylinders with a height of 70 Angstrom. One of the imposed fourfold axes running perpendicular to the plane of the crystalline layer is positioned at a protein-deficient region which forms a 25-Angstrom-wide pore. through the oligomer, (C) 1997 Academic Press.