Oral peroxidases (myeloperoxidase, sialoperoxidase) catalyze thiocyanate peroxidation into hypothiocyanite which is bacteriostatic or bactericidal against numerous bacterial species. NADH-hypothiocyanite-oxidoreductase is thought to protect bacteria which can express it; up to now, this enzyme activity was never purified. The present study analyzes, on one hand, the susceptibility of periodontal bacteria against hypothiocyanite and, on the other hand, proposes a purification design for the NADH-hypothiocyanite-oxidoreductase from Streptococcus sanguis, a commensal micro-organism of dental surfaces. The data suggest the importance of the bacterial biofilm on dental surfaces for production of antiseptic oxidants and for the control of their toxicity.