Preparative isoelectric focusing ( Rotofor) was introduced to carry out the prefractionation of a selenised edible mushroom Agaricus bisporus sample containing 1037 +/- 31 ( SD) mug Se g(-1). The water- soluble part of the sample was separated by the free solution apparatus into 20 fractions on the base of the pI values of the sample components. The separation was followed by the determination of total Se, soluble protein ( by Bradford method) and Se- speciation of the fractions, summed up in 14 pools according to the actual individual pH values. Proteolytic sample preparation was chosen prior to the speciation analysis carried out with the help of a HPLC- UV- HG- AFS system, applying cation and anion exchange chromatographies. Achieving 19% column recovery, the presence of Se( IV), selenomethionine, selenocystine and methylselenocysteine was detected, the latter for the first time in a mushroom sample.