Selenium plays an important role in scavenging active oxygen (AO) species as an essential constituent of glutathione peroxidase. On the other hand, several reports proposed a possible induction of toxic AO by selenium compounds in vitro. However, some of these experiments including ours, were revealed to conclude on the basis of experimental artifacts, and to have problems in the interpretation of data.Methods or principles so far used for the detection of AO species generated by selenium compound were measurement of chemiluminescence from lucigenin or luminol by AO species,the spectrophotometric analysis of reduction of ferricytochrome c or nitroblue tetrazolium (NBT) by superoxide anion (O2-), electron spin resonance (ESR) spectra using dimethylpyrroline oxide (DMPO) as a spin trapping agent, the deoxyribose decomposition by hydroxyl radical (HO), the salicylate hydroxylation by HO, and the strand breakage of DNA by AO. Many of these methods together with their principles seem to have some defects which prevent clear conclusion as stated below. (i) Lucigenin was found to mediate the formation of O2- in the presence of selenite and reduced glutathione (GSH). Therefore, lucigenin is not a suitable reagent. (ii) Luminol may also mediate O2- generation in the presence of HO. (iii) ferricytochrome c can be reduced to ferrocytochrome c in the mixture of selenite and GSH in the absence of oxygen. Moreover, the spectrophotometric method is interfered by turbidity of elemental selenium formed under some conditions in the reaction mixture containing selenite and GSH. (iv) NBT is also reduced by selenium compounds in the absence of O2.(v) ESR signals of AO species were obtained in the reaction mixture containing selenite and GSH, or in the solution of hydrogen selenide in the presence of O2. However, selenide decomposed spin adduct of DMPO with HO’ (DMPO-OH). Therefore, the intensity of the signals is not quantitative. (vi) CuZn-SOD is not necessarily a good tool to prov the involvement of O2-, because it enhanced HO generatdri in the reaction mixture containing selenite and GSH.Thus, we would like to emphasize that carefully designed experiments are required to further identify the molecular species of active oxygen induced by selenium compounds Selenium plays an important role in scavenging active oxygen (AO) species as an essential constituent of glutathione peroxidase. On the other hand, several reports proposed a possible induction of toxic AO by selenium compounds in vitro. However, some of these experiments including ours, were revealed to conclude on the basis of the experimental of artifacts, and to have problems in the interpretation of data. Methods or principles so far used for the detection of AO species generated by selenium compound were measurement of chemiluminescence from lucigenin or luminol by AO species, the spectrophotometric analysis of reduction of ferricytochrome c or nitroblue tetrazolium (NBT) by superoxide anion (O2-), electron spin resonance (ESR) spectra using dimethylpyrroline oxide (DMPO) as a spin trapping agent, the deoxyribose decomposition by hydroxyl radical (HO) the salicylate hydroxylation by HO, and the strand breakage of DNA by AO. Many of these methods together with their principles seem to hav (i) Lucigenin was found to mediate the formation of O2- in the presence of selenite and reduced glutathione (GSH). (ii) Luminol may (iii) ferricytochrome c can be reduced to ferrocytochrome c in the mixture of selenite and GSH in the absence of oxygen. Moreover, the spectrophotometric method is interfered by turbidity of elemental selenium formed under some (v) ESR signals of AO species were obtained in the reaction mixture containing selenite and GSH, or in the solution of Hydrogen selenide in the presence of O2. However, the selenide decomposed spin adduct of DMPO with HO ’(DMPO-OH). (vi) CuZn-SOD is not necessarily a good tool to prov theinvolvement of O2-, because it enhanced HO generatdri in the reaction mixture containing selenite and GSH.Thus, we would like to emphasize that carefully designed experiments are required to further identify the molecular species of active oxygen induced by selenium compounds