Mechanisms of resistance of Pseudomonas aeruginosa to 4-quinolones were investigated by examining the outer membrane proteins and DNA gyrases of resistant clinical isolates. Minimum inhibitory concentrations (MIC) were determined for fluoroquinolones, imipenem, β-lactams, gentamicin and tetracycline. Ciprofloxacin was the most active quinolone followed by ofloxacin and norfloxacin. Resistant clinical isolates had an MIC to ciprofloxacin of 4mg/l or greater, and resistance was shown to be stable after 20 passages on nutrient agar in all but one of the isolates. In several isolates cross resistance with imipenem and β-lactams was seen. All clinical isolates were resistant to tetracycline. Outer membrane profiles of sensitive and resistant strains of P.aeruginosa were examined and shown to be variable. In highly resistant strains, resistance to 4-quinolones could not be attributed to a particular membrane protein alteration as the isolates were not paired. Low level quinolone resistance in a series of paired isolates was linked with an increase in expression of a 48kD protein, and ceftazidime resistance with the appearance of a 42kD protein. These proteins were found to be non-covalently associated with peptidoglycan suggesting that they may function as porins. DNA gyrases from 22 clinical strains were isolated from a novobiocin/sepharose column and assayed by electrophoresis. The level where quinolones inhibited 50% of DNA gyrase supercoiling activity on relaxed pBR322 (IC₅₀), was determined for ciprofloxacin, ofloxacin and norfloxacin in all strains. In general the IC₅₀ concentrations were equivalent to or greater than, the MIC of each drug.