NAD (nicotinamide adenine dinucleotide) hydrolyzing enzyme is one of the extracellular enzymes and toxins produced by hemolytic streptococci. Although the function as a toxin is largely unknown, it has been suggested to be related to pathogenicity of acute infection. NADase is produced not only by Group A hemolytic streptococci but also by Group C and Group G strains. The amino acid sequences are highly conserved among them and the antibodies cross-react each other. Upon infection of hemolytic streptococci, the antibody titer to the NADase is increased similarly to anti-SLO (Strreptolysin O) antibody.