In Vivo, after administration of treatment, CONCENTRATIONs of antimicrobial agents will reduce to sub-inhibitory levels (sub-MIC) and may therefore affect the properties of target bacteria. The purpose of the present study was to investigate the in vitro effect of sub-minimum inhibitory CONCENTRATIONs (sub-MIC) of ampicillin, gentamicin and nalidixin acid on morphology, growth, ammonium production and urease activity of proteus mirabilis. Proteus mirabilis is well recognized as an important urinary tract pathogen.Materials and Methods: Several of its properties have been studied in relation to pathogenesis manifested in urinary tract such as urease activity.Results: In the presence of ampicillin, long filamenteous cells was produced and the length of the cells was increased at the higher CONCENTRATION of ampicillin. SubMIC of ampicillin and gentamicin affected the growth pattern and prolonged the lagphase of growth. This affected was significant when gentamicin was used. Nalidixic acid of 1/2 MIC greatly reduced the growth rate, while the lag phase was not changed. In the presence of sub-MICs of ampicillin and gentamicin, the amount of ammonium production increased. In medium with 1/2 MIC of ampicillin, the ammonium production was 30 times of control, while the urease specific activity of sonicated cells did not show any significant changes.Conclusion: It seemed that the antibiotics enhanced the cell membrane permeability for substrate and enzyme. Nalidixic acid didn"t show any significant effect on ammonia production, and urease specific activity of proteus mirabilis. The results indicate that the sub-MIC of antibiotics can effect virulence factors of proteus mirabilis.