Poly(hexamethylene guanidine hydrochloride) containing silicone reactive groups (PHGS) was used as an antimicrobial finishing agent for textile fabrics, and the effects of treatment methods and kind of fibers on the laundering durability of antimirobial activity were studied. The antimicrobial activity of PHGS-treated cotton fabrics was not affected by the treatment methods such as padding methods, padding temperature, drying or curing conditions. They showed excellent antimicrobial activity even after laundering with alkaline detergents. On the other hand the synthetic fiver fabrics, such as PET, nylon 6, and acrylics, treated with PHGS solution under various conditions exhibited no antimicrobial activity after launderings although they showed excellent antimicrobial activity before laundering. These results ruled out the possibility that the durability in the case of cotton fabrics may come from the formation of insoluble crosslinked film on the fiber surface via self-crosslinking between hydroxyl groups in PHGS. They rather confirmed the suggestion that the reactions between PHGS and hydroxyl groups in cellulose impart laundering durability to PHGS treated cotton fabric.