The present work highlights the high efficiency of silver nano-particles deposited over activated carbon, by electro-chemical deposition method, in controlling microorganisms in water. The anti-bacterial activity of the catalysts were determined qualitatively by testing the presence of coliforms in water after contacting with the catalyst, using a Readycult reagent. The catalytic characteristics of these materials are obtained by SEM (scanning electron microscopy), XRD (X-ray diffraction) and TPR (temperature programmed reduction). TPR results clearly indicate the presence of metallic silver in the dried catalyst prepared by electro-chemical deposition method and the presence of silver oxide and/or nitrate precursors in the catalysts prepared by impregnation method. SEM results indicate the presence of Ag particles in nanometer size. Comparison of the anti-bacterial activity of the Ag/C catalyst prepared by electro-chemical deposition method with that of the Ag/C catalyst prepared by conventional impregnation technique indicate that lower amount of former is sufficient in controlling the microorganism which is not the case with the latter. The main advantage of Ag/C catalyst prepared by electro-chemical deposition is that no pretreatment conditions like reduction are required for deactivation of microorganism in water, which is not the case with the catalysts prepared by impregnation technique. (C) 2004 Elsevier B.V. All rights reserved.