Summary The biosynthesis of seleno-analogues of sulfur amino acids has been studied in the ruminant (sheep) by repeated per os administration of H2Se75O3. In the biosynthetic processes of the rumen selenium can replace sulfur to form seleno-analogues of the sulfur compounds. Se75-selenocystine with cystine was chemically separated from wool hydrolysate. Chromatographic separation of the amino acids in wool is reported. Se75-selenocystine and Se75-selenomethionine in the wool hydrolysate was demonstrated. Neither selenocystine nor selenomethionine can be separated by present methods from their sulfur analogues.