Whole cells of Salmonella typhimurium were treated with Bacillus cereus phospholipase C or with CNBr-activated dextran. If phosphatidylethanolamine head groups are exposed and accessible on the outer surface of the outer membrane of these cells, it was expected that these groups would be hydrolyzed by the former agent, and become covalently coupled to the latter agent. With strains producing lipopolysaccharides of S or Rc type, results did not indicate the presence of any accessible head groups on the outer surface. In contrast, with strains that produce outer membranes containing less complete lipopolysaccharides (Rd or Re type) and reduced amounts of proteins, both methods clearly showed the presence of exposed phosphatidylethanolamine head groups. These data can be most easily explained by assuming that the outer membranes of S and Rc strains either contains all phospholipid molecules in its inner leaflet or has proteins that completely cover up the head groups at its outer surface. In either model, the reduction in the amount of outer membrane proteins in Rd or Re mutants would produce membranes with exposed phospholipid head groups. CNBr-activated dextran can be easily prepared, and reacts with high efficiency under near-physiological conditions. Its additional advantage as a nonpenetrating membrane-labeling reagent is that we can be quite confident on its impermeability because of its size, in contrast, with most other reagents whose presumed impermeability is dependent only on the presence of charged groups.