The bactericidal activity of piperacillin was evaluated by the kill rate kinetics method. Piperacillin compared with amoxicillin, amdinocillin, mezlocillin, cefotaxime, ceftazidime and latamoxef. Against E. coli, piperacillin and antibiotics electively bound to penicillin-binding protein (PBP) 3 had the same, mainly time-dependent, bactericidal activity. Antibiotics bound to PBP 1 and 2 mainly have a dose-dependent activity. The bactericidal activity of piperacillin against E. coli was enhanced when the drug was combined with amoxicillin and amdinocillin; in contrast, the kill rate remained unmodified when piperacillin was combined with latamoxef. When the piperacillin-amikacin combination was tested against Pseudomonas aeruginosa, Enterobacter cloacae, Escherichia coli and Serratia marcescens, early synergism (5th hour), was weak, but after 24 hours piperacillin reduced the regrowth observed with the aminoglycoside, and synergism was much more pronounced irrespective of the species investigated.