Amino acids present in blood plasma may be targets for oxidation and chlorination by HOCl/OCl(-). N-Chloroamino acids have been reported to be less reactive, but more selective than HOCl/OCl(-) in their reactions; therefore, they may act as secondary mediators of HOCl/OCl(-)-induced injury. This study compared the effects of five N-chloroamino acids (AlaCl, LysCl, SerCl, AspCl and PheCl) on erythrocytes with the action of HOCl/OCl(-). The N-chloroamino acids differed in stability and reactivity. They had a weaker haemolytic action than HOCl/OCl(-); HOCl/OCl(-), AlaCl and PheCl increased osmotic fragility of erythrocytes at a concentration of 1 mm. Oxidation of glutathione, formation of protein-glutathione mixed disulphides and efflux of GSSG from erythrocytes were observed for erythrocytes treated with all the employed chloroderivatives, while increased oxidation of 2',7'-dichlorofluorescin was detected only after treatment of the cells with 1 mm HOCl/OCl(-), AlaCl and PheCl. Generally, the reactivity of at least some N-chloroamino acids may be not much lower with respect to HOCl/OCl(-).