Cell envelope membranes were isolated from Pseudomonas aeruginosa. These membranes were resolved into cytoplasmic membrane rich and outer (cell wall) membrane rich fractions by discontinuous sucrose density gradient centrifugation. The resolution was based on the separation of enzyme activities and 2-keto-3-deoxyoctonate. Analysis by gel electrophoresis revealed that two of the three major cell envelope protein components were found in the fraction rich in outer (cell wall) membrane. These two protein components were previously shown to occur in a protein–lipopolysaccharide complex in this microorganism.