Selenium speciation in selenium-enriched Bifidobacterium animalis 01(B.animalis 01)has been studied.Firstly,B.animalis 01 accumulated inorganic selenium from culture medium.Secondly,nucleic acid,polysaccharide,and proteins were separated from Se-enriched cells respectively.Finally,Protein was separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis(SDS-PAGE).The selenium concentration of tested samples was determined by hydride generation-atomic absorption spectrometry(HG-AAS).The results showed that B.animalis 01 could absorb 16.7%-39.6% of inorganic selenium in the medium and transform most of it into organic selenium.The proteins in Se-enriched B.animalis 01 showed the strongest ability to incorporate selenium.The organic selenium content of the protein,polysaccharide and nucleic acid extractions was in the following order:protein-bound Se > polysaccharide-bound Se > nucleic acid-bound Se.In the proteins containing selenium,the molecular mass of most proteins or protein subunits containing selenium was about 10-20 ku