There was an increase in the resistance of clinical isolates to piperacillin as compared to that in 1987-1988. It was shown on the kinetic models that there was difference in the effect of the inhibitory and subinhibitory concentrations of the drug on microorganisms in various growth phases. No postantibiotic action against Staphylococcus aureus and Pseudomonas aeruginosa and a slightly pronounced dose-dependent effect with respect to Escherichia coli were noted.