N-chlorotaurine (NCT) is known to play an important role in the human defence system. The already proved utility of the sodium salt as a disinfectant in human medicine suggested a thorough investigation of its chemical properties. Chlorine transfer to N-H groups (transhalogenation) and oxidation of thio and aromatic compounds represent its main reactions. Auto-chlorination causes disproportionation forming N, N-dichlorotaurine (NDCT) with K(d) = [NDCT][taurine]/f(a)[NCT]2aH+ = (4.5 +/- 0.8) x 10(6), while the reaction with ammonium releasing NH2Cl is characterised by K(NHCl2) = [NH2Cl][taurine]/[NCT][NH4+]f(a)2 = 0.02 +/- 0.004. The verified unique stability and low level reactivity of NCT are considered essential for its function in the mammalian defence system and its practical applicability, which manifests itself in an optimal compromise between microbicidal activity and toxicity.