The in vitro activities of azlocillin, carbenicillin, ceftriaxone, piperacillin, N-formimidoyl thienamycin (N-f thienamycin) and tobramycin have been compared against clinical isolates of Pseudomonas aeruginosa (n = 100) and Acinetobacter calcoaceticus (n = 50). An agar dilution method was employed for measurement of minimal inhibitory concentration (MIC). Tobramycin was the most active drug against P. aeruginosa (MIC≤2 mg/l). Of the β-lactam antibiotics, N-f thienamycin showed the highest activity, and carbenicillin was least active. Both N-f thienamycin and tobramycin were highly active against A. calcoaceticus (MIC≤ mg/l), although one isolate was resistant to tobramycin (MIC > 16 mg/l). The other drugs were only moderately active against A. calcoaceticus.