Previous studies reported that, at neutral pH (e.g. 7.4), many preservatives do not have antimicrobial effect against Pseudomonas aeruginosa and some strains of E. coli . This study investigated the effect of combination of low pH and some preservatives (e.g. sodium chloride, potassium sorbate or Thymus vulgaris extract) on the growth and caseinase production of Pseudomonas aeruginosa and E. coli at 31 degrees C and 7 degrees C. At 31 degrees C, although growth of the strains was not affected by low pH (5.7) alone, caseinase production by both strains was decreased. A combination of low pH and NaCl, potassium sorbate or thyme extract significantly reduced (p < 0.05) growth and caseinase production by E. coli 28 at 31 degrees C. For P. aeruginosa , addition of NaCl or thyme extract to nutrient broth (low pH) did not affect the growth but reduced caseinase production of the tested strain. Combination of low temperature (7 degrees C) and growth in nutrient broth (low pH) plus sodium chloride or nutrient broth (low pH) plus potassium sorbate was the most effective treatment in reducing or inhibiting growth and caseinase production of the tested strains.