14C-labelled cellulose was added to seven different soils containing silt + clay (particles <0.02 mm) in amounts which varied from 8 to 75 per cent. The cellulose was allowed to decompose, and the amounts of labelled C transformed into metabolites hydrolyzable into amino acids were determined. The amounts of labelled amino acid C in the soils were proportional to their content of silt + clay. After 30 days of incubation labelled amino acid C remaining in the soil with the lowest content of silt + clay constituted 6 per cent of the carbon added in cellulose, as compared with 18 per cent in the soil with the highest content of silt + clay. These values had decreased to 5 and 13 per cent respectively after 2 years of incubation. The order between the soils in the content of labelled amino acid C established during the first month of incubation, was thus roughly maintained throughout the period of incubation. The biological half-life of the labelled C in amino acids varied in the seven soils during the last year of incubation from 3 to 8 years. The variation was, however, not related to the amount of silt + clay. n the soils had been incubated with the labelled material for 2 years, samples of the soils were exposed to “stress” treatments: air drying-rewetting; increased biological activity caused by addition of glucose, and exposure to chloroform vapour. The treatments resulted in an evolution of labelled C in CO, which was 5–10 times larger than the evolution from untreated samples. The increase in the CO2 evolution caused by the treatments in the different soils was, however, not related to the amount of silt + clay, and a high content of this material did not protect organic material against the effect of the treatments. is concluded that the silt + clay fraction ensures stabilization of amino acid metabolites produced during the period of intense biological activity that follows the addition of decomposable, energy rich material to the soil. The amount of amino acid metabolites stabilized increased with increasing concentration of silt + clay, but the rate of decay of the amino acid material during later stages was largely independent of the concentration of silt + clay.