Tazobactam (Tazosyn®) is a novel β-lactamase inhibitor belonging to a class of penicillanic acid sulfones. Tazobactam was developed to be used in combination with piperacillin against β-lactamase producing microorganisms. The current study was conducted to determine the effect of tazobactam on the pharmacokinetics and the antibacterial activity of piperacillin in dogs. Three groups of animals consisting of three beagle dogs per group were used for the study. The animals were administered a single I.V. dose of tazobactam (40 mg/kg), piperacillin (320 mg/kg) or the combination in a ratio of 1:8 (tazobactam: piperacillin). Blood samples were drawn at different time intervals. The serum bactericidal titers (SBT) were determined against a plasmid-mediated β-lactamase producing strain of Eschericia coli (LSU-80-8). Serum concentrations of both compounds were determined by high performance liquid chromatography. The mean serum bactericidal titers of the combination was 1:16 against this piperacillin resistant strain of E. coli, 2 h after dosing as compared to less than 1:2 when piperacillin was given alone at the same dose. This indicates that serum concentrations greater than 187 μg/ml of piperacillin (SBT<1:2) were required to kill 99.9% of the piperacillin resistant E. coli inoculum (10⁶ CFU/ml) when piperacillin was given alone. However, when piperacillin was administered in combination with tazobactam, concentrations as low as 7 μg/ml of piperacillin and 2 μg/ml of tazobactam were sufficient to exhibit the same bactericidal activity. These results indicate an in-vivo synergistic effect of tazobactam on the piperacillin activity at this dosing ratio which lasted for approximately 5 h after dosing. The coadministration of tazobactam did not appear to affect the pharmacokinetic parameters of piperacillin. However, the elimination of tazobactam was significantly inhibited when coadministered with piperacillin, resulting in a reduction of the clearance (3.5 vs. 7.5 ml/min per kg) and prolongation of the half-life (43 vs. 31 min).