A Pseudomonas aeruginosa strain (P.aeruginosa), recently isolated in clinical practice, was tested to evaluate the changes induced in the minimal inhibitory concentration (MIC) values and in the outer membrane (OM) components by a five-day exposure to sub-MIC concentrations of the quinolones ciprofloxacin and PD-131.628 and the carbapenems imipenem and meropenem. The treated strain showed a thirty two-fold increase in MIC values for quinolones and a sixteen-fold increase for carbapenems. The electrophoretic profile of the OM proteins of the strain treated with quinolones showed that ciprofloxacin induces loss of the 47 Kd protein band, whereas PD-131.628 modifies the protein pattern of the strain only after five days of exposure. The carbapenems engendered disappearance of the same protein band. Qualitative lipopolysaccaride (LPS) analysis did not reveal any change after antibiotic treatment of the strain, whereas the 2-keto-3-deoxyoctonic acid (KDO) assay showed considerable reduction in the strain treated with sub-MIC doses of meropenem. It can therefore be safely stated that the D2 protein plays an important though not exclusive role in enhancing strain resistance against the two classes of antibiotics tested in our study.