1. An anaerobic strain of Clostridium tertium was isolated from the soil which produced an enzyme which, acting on O(H) substance and group O red cells, abolished not only O(H) activity but also cross-reactivity with anti-pneumococci Type XIV chicken serum. The enzyme preparation decomposed O(H) substance, converting it into a substance which contained D-galactosamine and/or D-glucosamine as antigenic determinant. This enzyme preparation also decomposed Lea substance. In addition to this enzyme, the strain produced A-decomposing enzyme.2. These enzymes were found in a fraction which precipitated at 40-50% ammonium sulfate saturation of culture filtrate of the strain. By means of DEAE cellulose column chromatography, the enzyme decomposing O(H) and Lea could be separated from the A-decomposing enzyme. For both of them, the optimal pH ranged 6.8-7.2, the optimal temperature 30-37°C, and after heating at 100°C. for 5 minutes, the enzymes were inactivated.3. The actions of these enzymes were inhibited by such metal salts as mercuric chloride and copper sulfate. The enzymic action to decompose O(H) substance reacting with anti-O(H) antibody in eel serum was inhibited strongly by L-fucose and slightly by lactose and D-galactose. The enzymic action to decompose O(H) substance reacting with anti-O(H) antibody in anti-Sh. dysenteriae chicken serum was inhibited strongly by D-galactose, lactose, D-galactosamine, and N-acetyl-D-galactosamine, and slightly by L-fucose. And the enzymic action to decompose O(H) substance reacting with antibody in anti-pneumococci Type XIV chicken serum was inhibited by D-galactose, lactose, N-acetyl-D-glucosamine, α-ethyl-N-acetyl-D-glucosaminide, D-glucosamine and L-fucose. The action of Lea decomposing enzyme was inhibited strongly by D-galactose and L-fucose, and also inhibited by lactose, melibiose, N-acetyl-D-glucosamine, α-and β-ethyl-N-acetyl-D-glucosaminide and N-acetyl-D-galactosamine. And the action of the A-decomposing enzyme was inhibited most strongly N-acetyl-D-galactosamine, less strongly by lactose and D galactose, and slightly by N-acetyl-D-glucosamine.