Antimicrobial activity of sucrose monolaurate (SL) in the range of pH 5.0 to 7.0 was tested on bacteria in nutrient broth. The activity against Bacillus licheniformis, Bacillus subtilis and Staphylococcus aureus increased at low pH. S. aureus was used to determine the antimicrobial effect of SL. During pretreatment with SL, we observed that the lag phase of S. aureus could be prolonged by the use of SL at high concentration and low pH.When the cells of S. aureus were treated with SL at pH 5.0, increased leakage of Mg2+ and ninhydrin-positive substances was observed. In contrast, increased leakage of proteins and UV-absorbing substances was observed at pH 7.0.We assumed that antimicrobial action of SL on S. aureus was bacteriostatic, and that the pH dependence of antimicrobial action was related to the leakage of Mg2+ and ninhydrin-positive substances.