Abstract: Aim To study the postantibiotic effects(PAEs)in vitro induced by three penicillins.Methods The PAEs induced by Cloxacillin (CXL), Amoxacillin(AM)and Mezlocillin (MZL)against three species of bacteria were determined by spectrophotometric method.Results Marked PAEs were found, even in the sub-MIC concentration of these penicillins and there was a strong concentration-dependence.Negative PAEs were defined, however,when AM and MZL were used against gram一negative bacilli.Conclusion The results suggested that longer dosing interval, especially when penicillins were used against gram -positive coccus, should be recommended.

Key words: postantibiotic effects, penicillin, spectrophotometric method