Bio shield AM 500 3- (trimethoxy silyl)- propyl - dimethyl octadecyl ammonium chloride was applied to cotton fabrics by a cold pad-batch method in the presence of an alkaline catalyst to evaluate its use as a durable antimicrobial finish. The antimicrobial activities of the Bio shield AM 500 treated cotton fabrics were evaluated quantitatively against AATCC 147 by Gram staining and Turbidity test method. The turbidity value was maintained over 187 even after being exposed to 15 consecutive home laundering conditions. The antimicrobial treated sweat sample was tested for microbial activity and the turbidity was 189.