Actinobacillus suis secretes metalloproteases into its medium. These secreted proteins, when concentrated by precipitation with 70% (NH4)(2)SO4 or methanol, displayed proteolytic activity at >200 kDa molecular mass bands in 10% polyacrylamide gels copolymerized with bovine casein (1%). They showed activity in a broad pH range (from pH 5 to pH 10) and were inhibited by 20 mm EDTA or EGTA, but could be reactivated by calcium. They were found heat stable at 40degreesC, 50degreesC, 60degreesC, and 70degreesC, but their activity diminished at 80degreesC or higher. They degraded pig and bovine IgG and cross-reacted with a polyclonal serum against a high molecular mass secreted protease from A. pleuropneumoniae. Extracellular proteases could play a role in diseases caused by A. suis.