To endow fabrics with an antibacterial property, polyethyleneimine (PEI), which has a strong cationic characteristic, was physically immobilized onto a poly (ethylene terephthalate) (PET) fabric and the antibacterial activity of the modified fabric was tested using two methods. Antibacterial modification of the fabric was easily accomplished by simply immersing it in PEI solution for 1 min and the subsequent post-treatment for 3 h at 60°C. The amount of the immobilized cation increased with the molecular weight and the concentration of PEI. Wettability and lubricity of the PET fabric were slightly improved by this PEI adsorption. The PET treated with PEI of molecular weight of 75, 000 at 1 wt% concentration had excellent antibacterial activity for Staphylococcus aureus . In addition, an antibacterial effect of the PET fabric was also observed for Klebsiella pneumoniae and MRSA. The antibacterial activity of the PEI-treated fabric was maintained even after being washed ten times. Two different antibacterial tests indicated that the antibacterial event occurred upon direct contact of the fabric surface with bacteria without any significant fear of destruction of normal flora. A skin irritation test using a cultured human skin model showed the modified fabric to be negative.