1. In studying the origin of nitrogen in humus formation it might be valuable to consider the conversions of amino acids in soil. 2. A survey was made of the literature on the presence and on the conversions of soil amino compounds. Speculations on the character of the resistant soil amino acid fraction have been given. 3. The soil was incubated with amino acids and conversions of amino acids were studied respirometrically. Oxygen consumption and carbon dioxide evolution were determined, and the content of amino acids, keto acids, ammonia and nitrate in aqueous or 1 N sodium chloride extracts. 4. The amino acids disappeared rapidly, mineralisation being completed within a few days. The ratio of the mineralisation products remained fairly constant during the period of investigation. When aeration was sufficient, small differences in moisture content gave no rise to alterations in the mineralisation yields. Under similar conditions the nitrification was enhanced by increasing moisture content. Storing the soil sample in the laboratory under constant temperature and moisture content did not give rise to differences in the mineralisation yield, but to a marked depression in the rate of nitrification. The relative carbon mineralisation yield was independent of the concentrations of amino acid administered; the relative nitrogen mineralisation yield increased with enhancing concentration. Amino acids with a high C/N-ratio give rise to lower nitrogen mineralisation percentages than those with low values. 5. During the decomposition of amino acids in soil no significant amounts of keto acids could be detected. The keto acids were broken down more rapidly than the corresponding amino acids. Their carbon mineralisation percentages were slightly lower than with amino acids. This might be due to a concentration effect, as the keto acids in the experiments mentioned had a 100 times higher concentration. It is quite reasonable to assume that keto acids are intermediates in amino acid breakdown in soil. 6. The differences between the laboratory conditions and those in nature, and the effect on the results of the experiments are discussed. No losses of substrates or of reaction products occur by denitrification, volatilisation of ammonia, nor by irreversible fixation of ammonia and of amino acids to soil colloids. The amino acids are deaminated oxydatively; the nonmineralised fraction has a remarkably constant C/N-ratio. This accounts for most of the amino acids investigated. The assumption of Swaby that the amino acids in organisms that had just died would combine with quinones also becoming available may explain most of the properties of the resistant soil amino acid fraction.