The effectiveness of an aqueous ozone treatment in reducing Escherichia coli O157:H7 and Salmonella serotype Typhimurium on hot carcass surfaces was determined with the use of a model carcass spray cabinet. Carcass surface regions were removed from carcasses and inoculated with feces containing 10(6) to 10(7) CFU each of E. coli O157:H7 and Salmonella Typhimurium per g and were then exposed to a water wash or to a water wash followed by a sanitizing ozone treatment. Water washes were applied at 28 degrees C beginning at a pressure of 10 lb/in2 and gradually increasing to 400 lb/in2. Ozone treatment was carried out by spraying surfaces with an aqueous ozone solution (80 lb/in2 at 28 degrees C) containing 95 mg of ozone per liter. Pathogen reductions achieved with ozone treatment were not significantly different from those achieved with a water wash alone. In addition, ozone treatment did not reduce E. coli O157:H7 or Salmonella Typhimurium contamination that was spread over the carcass surface as a result of the water wash. Under the conditions of this study, the aqueous ozone treatment applied resulted in no significant improvement over a water wash in reducing pathogens on beef carcass surfaces.