The impact of stormwater runoff to receiving water bodies is a growing concern across the nation. An elevated microbial concentration at recreational sites after rainstorms is a common phenomenon, and it poses significant threats to the safety of beach-goers. Local authorities seek an effective measure for reducing microbial concentration from storm sewers; although the development of such technological options is urgently needed, it remains challenging. A pilot-plant study was conducted to evaluate the microbial reduction efficacy of the Smart Sponge^® Plus, a filter-type antimicrobial stormwater treatment material developed by AbTech Industries, Inc. Two indicator organisms, Escherichia coli and enterococci, were tested under different flow rates, material packing densities, and influent microbial concentrations in the fall of 2007. The test results exhibited an excellent removal capacity of over 95% for enterococci, regardless of the operating factors mentioned above; for E. coli, although for a lesser extent, an average of 47% was achieved. No particular correlation between operational factors and removal efficiencies was found for both species, due to the limitation of data quantity. However, an important implication was obtained through this study about pretreatment condition of the Smart Sponge^® Plus media; to verify the findings, further tests are needed.