We studied the behaviour of 56 clinical isolates of Pseudomonas aeruginosa strains against the following beta-lactam antibiotics: cefotaxime, cefoperazone, cefsulodin, lamoxactam, Ro 13-9904, azlocillin, mezlocillin and ticarcillin. Twenty-six strains were resistant to gentamicin and 30 to gentamicin and/or carbenicillin. The MICs were measured by the serial dilution test on solid agar. Cefsulodin was the most active cephalosporin against carbenicillin-resistant strains (MIC greater than or equal to 128 mg/l); it inhibited 56.6% of these strains at a concentration of 8 mg/l. Azlocillin was the most active penicillin, inhibiting 79.96% of the same strains at a concentration of 64 mg/l. Cefsulodin was the most active cephalosporin against the gentamicin-resistant group of Pseudomonas aeruginosa strains (MIC greater than or equal to 8 mg/l) which were sensitive to carbenicillin (MIC less than or equal to 64 mg/l). It inhibited 100% of the strains at a concentration of 4 mg/l. All of the penicillins studied inhibited all of the strains in this group. The required concentrations were the following: 16 mg/l for azlocillin, 32 mg/l for mezlocillin and 64 mg/l for ticarcillin.