The level of mineral utilization depends on many factors related to animals as well as to the chemical form of given nutrients. It has been experimentally demonstrated that animals utilize inorganic forms of minerals less efficiently than organic ones. Hence, an attempt has been made to evaluate the bioavailability of selenium bound in different compounds to lambs. Selenium supplementation in fodder mixtures was another aspect included in our tests. Thus, an experiment was conducted on 48 growing lambs, testing the level and chemical forms of selenium in fodder mixtures for animals. In group I (the control) lambs received fodder mixture without any selenium supplement. In the experimental groups, selenium was supplemented as sodium selenite in the amount 0.2 mg Se kg-1 d.m. feeds (II), or selenium enriched yeast (Se-yeast) in the amounts of 0.1 and 0.2 mg Se kg-1 d.m. of feed groups III and IV, respectively. At the end of the experiment, when animals had reached 35 kilos of weight, blood samples were taken. The activity of glutathione peroxidase was estimated in heparinized blood samples. Eight lambs were chosen from each group and killed to collect samples of soft tissues (liver, kidney, brain, muscle). The content of selenium was determined in the tissue samples. Supplementation of feeds for lambs with selenium, both inorganic (sodium selenite) and organic (Se-yeast), increased the content of the element in soft tissues of animals. The highest level of the element was found in the liver and kidneys: 4.65 and 4.90 and 2.10 and 2.30 mg kg-1 fresh tissue, of the lambs receiving sodium selenine and Se-yeast in the amount 0.2 mg Se kg-1 D.M. of feed (groups II and IV), respectively. Selenium compounds added to feeds given to lambs significantly (P£0.01) increased activity of GSH-Px in blood, especially in the case of animals which received yeast enriched with selenium. In blood of the lambs which received feeds with Se-yeast in the amount 0.2 mg Se kg-1 d.m. of feed (group IV), the activity of glutathione peroxidase was by 129.71 U gHb-1 higher, and of the animals which received Se-yeast in the amount 0.1 mg kg-1 d.m. or sodium selenite, the activity of the enzyme was higher by 86.33 and 86.35 U gHb-1, respectively, than the activity of this enzyme in blood of animals from the control group. Supplementation of lambs’ rations with Sethe form of selenite or yeast enriched with selenium forms increased the content of Se in soft tissues and gluthatione peroxidase activity in comparison with animals which did not receive additional doses of this nutrient in fodder mixtures. The availability of Se was more profoundly affected by the amount of the element added rather than its form.