Lipopolysaccharides extracted from Pseudomonas aeruginosa strain K799 and its antibiotic-supersusceptible derivative Z61 were analyzed chemically and chromatographically. The side-chain polysaccharides purified by gel exclusion chromatography were compositionally identical, being composed of fucosamine (2-amino-2,6-dideoxygalactose), quinovosamine (2-amino-2,6-dideoxyglucose), and an unidentified amino sugar. In addition, low amounts of the core-specific components (glucose, rhamnose, alanine, and galactosamine) were found associated with the side chains from both strains. An average molecular weight of 38,000 to 50,000 was calculated for this fraction based on the glucose and rhamnose levels. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that the lipopolysaccharides from these two strains were microheterogeneous. Qualitative analysis of the lipopolysaccharide neutral sugars, using a series of single-step revertants of mutant Z61, demonstrated that full revertants showed patterns indistinguishable from those of the wild-type strain K799, whereas partial revertants had intermediate levels and mutant Z61 low levels of neutral sugars. Quantitative analysis revealed that the core oligosaccharide fraction from the wild-type strain had a glucose/rhamnose/galactosamine ratio of 4:1:1, whereas the core from Z61 exhibited major deficiencies in both glucose and rhamnose. The lipid A from both strains contained five fatty acids, namely, 3-hydroxydecanoate, dodecanoate, 2- and 3-hydroxydodecanoate, and hexadecanoate. Whereas the overall fatty acid content was equal, the mutant strain showed markedly lower levels of dodecanoate and hexadecanoate and increased levels of 2-hydroxydodecanoate. Results of whole-cell fatty acid analyses were consistent with this observation. Evidence for an additional alteration of the lipid A of strain Z61 was obtained from acid hydrolysis studies and infrared spectra of isolated lipid A, although the actual chemical basis could not be determined by a variety of techniques. It is suggested that the state of the lipopolysaccharide is able to influence the number of open functional protein F pores in the outer membrane of P. aeruginosa.