Baker's yeast is able to assimilate carbon, nitrogen, phosphorus and sulphur sources together with a great number of minerals and trace elements into a palatable, nutritious product. The metabolism of yeast is precisely controlled during the production growth phase and thus it is possible to determine the composition of the product by controlling the raw materials. Because of existing deficiencies in the availability of certain trace elements, mainly selenium, in Finnish diets, we started testing the possibilities for enriching yeast with this essential trace element about five years ago. We have succeeded in developing a special yeast product with a selenium concentration of 500 mg/kg dry matter. Selenium was expected, because of its structural similarity to sulphur, to replace sulphur in the biosynthetic reactions of the yeast cell. We have recently studied the incorporation and distribution of selenium in yeast with radioactive selenium (75Se). Analysis of the protein fraction of selenium yeast has shown that selenium is present in all the major soluble proteins. Selenomethionine was identified as the major selenium-containing compound in the protein fraction as well as in the whole cell.