A selective method for determining the primary amino group in certain d -α-amino acids on the 0.1-meq. scale is described. The sample is treated with d -amino acid oxidase and catalase at pH 8.3 for 60 to 90 min. at 35 to 37.5 °C. The ammonia formed is steam-distilled in presence of a borax-potassium carbonate mixture, collected in 2% boric acid solution, and titrated with 0.01 N hydrochloric acid. Seven aliphatic α-amino acids, two aromatic α-amino acids, and tryptophan gave results accurate to about 5% relative. The presence of large excess of the l -isomers does not affect the analysis. No ammonia is produced from α-amino acids which are not resolvable. Serine, threonine, and glutamic acid gave low results. Lysine and histidine are not attacked by d -amino acid oxidase.