Background: We compared the antibacterial effect of piperacillin, piperacillin-tazobactam and imipenem in a paired rat thigh abscess model. Methods: Two abscesses were provoked in the thighs of rats, one on the right with an extended-spectrum beta-lactamase (OXA-14)-producing Pseudomonas nosa (Ps-162) and the other on the left thigh with a control strain. Results: The colony counts from the abscesses in log 10 colony-forming units per gram (mean +/- SD) in the imipenem group were 2.78 +/- 1.71 and 3.19 +/- 1.66, in the piperacillin-tazobactam group 4.36 +/- 0.23 and 2.44 +/- 1.97, and in the piperacillin group 4.44 +/- 0.21 and 3.71 +/- 0.99 for Ps-162 and the control strain, respectively. The mean colony counts were different (p < 0.05) between Ps-162 and the control strain in the piperacillin and piperacillin-tazobactam groups. Conclusion: These data showed that piperacillin and piperacillin-tazobactam were significantly less effective against extended-spectrum beta-lactamase-producing P. aeruginosa, while imipenem was equally effective against both Ps-162 and the control strain in this abscess model. Copyright (C) 2001 S. Karger AG, Basel.