Carbon-14 labelled cellulose or glucose were added to a medium loam and two sandy soils. The soils were incubated at 20°C for about 6 yr under laboratory conditions. Six to 12 per cent of the labelled carbon added to the soils was transformed into metabolites hydrolysable to amino acids during the first 10–30 days of the incubation period. The newly-formed metabolites decayed slowly as incubation continued. During the first 100–300 days of the incubation, losses of labelled amino acid carbon from soil were curvilinear when plotted on a semi-logarithmic scale. After this time the decay curves became linear, indicating half lives of 6–7 yr for the labelled carbon in amino acids. However, the labelled amino acid metabolites decayed at a faster rate than the native amino acid compounds of the soil, since they constituted a gradually decreasing percentage of total soil amino acid carbon as incubation proceeded. Twenty-six to 30 per cent of the total labelled carbon remaining in the soils after 6 yr of incubation was located in amino acids when the labelled carbon was added as cellulose, compared to 43 per cent when the labelled carbon was added as glucose. The amounts of amino acid metabolites extracted by sodium hydroxide or by the chelating ion-exchange resin Dowex A-1 decreased during the period of incubation. The unlabelled soil carbon as a whole was more extractable by the resin treatment than the labelled. Sixteen protein amino acids and two amino sugars were detected in hydrolysates of soils that had been incubated with either labelled cellulose or glucose for 6 yr. Each of these compounds was isolated and found to contain labelled carbon.