Active oxygen species or free radicals are considered to cause extensive oxidative damage to biologica! mac- romolecules, which brings about a variety of diseases as well as aging. The idea] scavenger for active oxygen should be ‘active hydrogen‘. ‘Active hydrogen' can be produced in reduced water near the cathode during electrolysis of water. Reduced water exhibits high pH, low dissolved oxygen (DO), extremely high dissolved molecular hydrogen (DH), and extremely negative re- dox potential (RP) values. Strongly electrolyzed-re- duced water, as well as ascorbic acid, (+)-catechin and tannic acid, completely scavenged 03 produced by the hypoxanthine-xanthine oxidase (HX-XOD) system in sodium phosphate buffer (pH 7.0). The superoxide dis- mutase (SOD)-like activity of reduced water is stable at 4°C for over a month and was not lost even after neutralization, repeated freezing and melting, defla- tion with sonication, vigorous mixing, boiling, re- peated filtration, or closed autoclaving, but was lost by opened autoclaving or by closed autoclaving in the presence of tungsten trioxide which efficiently ad- sorbs active atomic hydrogen. Water bubbled with hy- drogen gas exhibited low DO, extremely high DH and extremely low RP values, as does reduced water, but it has no SOD-like activity. These results suggest that the SOD-like activity of reduced water is not due to the dissolved molecular hydrogen but due to the dissolved atomic hydrogen (active hydrogen). Although SOD ac- cumulated H20; when added to the HX-XOD system, reduced water decreased the amount of H30; produced by XOD. Reduced water, as well as catalase and ‘To whom correspondcncc should be addresscd. Fax: 81-92-642- 3052. E-mail: sirahulafl“grt.kyushu-u.ac.jp. Abbrcviutionsz AET, 2-(aminocthyl)isothiurunium; AsA, ascorbic acid; CL, chemilumincsccnce; CLA. C ypn'dina luciferin analog; DO, dissolved uxygcn; DH. dissolved hydmgcn. EC. elettrica] conduc- tunce; HX, hypoxanthinc; RP. redox polential; SOD. supemxide dis- mulase; XOD. xunthim: uxidmxe.