Twenty Swedish Landrace lambs were divided randomly into two equal groups, group I and group O. Group I received approximately 0.1 mg selenium as sodium selenite per day and group O 0.1 mg selenium as selenium in yeast per day. The animals were fed per day 1.2-1.5 kg of hay and 0.3 kg of a commercial concentrate supplemented with vitamins and minerals. The trial lasted 3.5 months. As monitored by micro particle induced X-ray emission (mu-PIXE), the number of selenium bearing erythrocytes increased faster in group O than in group I indicating some difference in the metabolism of inorganic and organic selenium. During the trial the concentration of sulphur decreased in the erythrocytes in group O. The reason for this is not known. The zinc and copper concentrations of individual erythrocytes were unaltered. The blood GSH-Px activity increased at the same rate in the two groups indicating that selenium as sodium selenite and selenium in yeast was about equally available. The rate of growth in the two groups was almost the same and a significant difference in the final weight was not observed.