Carbenicillin resistance in strains of Pseudomonas aeruginosa isolated in Britain is mediated more frequently by "intrinsic factors" than by beta-lactamase production. Intrinsically carbenicillin-resistant isolates almost invariably were more resistant to azlocillin, cefoperazone, cefotaxime, ceftazidime, chloramphenicol, tetracycline and nalidixic acid than were carbenicillin-susceptible strains. This cross-resistance to different classes of antimicrobials suggested an impermeability-based mechanism of resistance, perhaps involving the outer membrane. The structure and composition of the outer membrane of the pseudomonas cell also influences the O-serotype specificity and the susceptibility to many bacteriophages. We therefore examined these properties for possible relationships to antibiotic resistance. Carbenicillin-resistant (122) and -sensitive (127) P. aeruginosa strains from 24 hospitals were compared. Serotype O:1, O:3, O:6, O:10 and O:11 strains predominated in both groups. Non-typable and polyagglutinating strains were infrequent in both groups. With one possible exception, none of 18 bacteriophages showed a significant preference for carbenicillin-resistant or -sensitive strains. Variation between strains was observed in the electrophoretic profile of LPS and this could be related in part to serotype, but not to antibiotic resistance. Our results contrast with those of earlier small-scale studies which have claimed relationships between surface properties and antibiogram in P. aeruginosa, and suggest that interpretation of the minor changes in LPS sometimes observed in association with the development of antibiotic resistance in vitro requires caution.