The outer membranes of Gram-negative bacteria represent selective, permeability barriers to environmental molecules. This function is accomplished in two ways. First, outer membranes exclude many larger hydrophilic molecules, including enzymes and other proteins, and most hydrophobic ones, by virtue of their unique composition. It has been demonstrated in Escherichia coli that outer membranes are asymmetric bilayers containing a unique species of glycolipid, lipopolysaccharide (LPS), in its outer leaflet, and phospholipids in its inner leaflet. The highly negatively charged LPS is stabilized by divalent cations. The observations that Helicobacter outer membrane proteins are Triton X-100 insoluble in the absence of EDTA or NaCl (1,2), that Helicobacter contains substantial amounts of LPS 3, and that Helicobacter is resistant to highly hydrophobic antibiotics, such as trimethoprin and nalidixic acid (4), lead one to believe that this organism is thematically similar.