14C-labeled cellulose and 15N-labeled (NH4)2SO4 were added to 4 soils with clay contents of 4, 11, 18 and 34%, respectively. Labeled cellulose was added to each soil in amounts corresponding to 1, 2 and 4 mg Cg-1 soil, respectively, and labeled NH4+ at the rate of 1 mg N/25 mg labeled C. After the 1st mo. of incubation at temperatures of 10, 20 and 30.degree. C, respectively, 38-65% of the labeled C added in cellulose had disappeared from the soils as CO2, and 60-nearly 100% of the labeled N added as NH4+ were incorporated into organic forms. The retention of total labeled C was largest in the soil with thg highest content of clay where after 4 yr it was 25% of that added, compared to 12 in the soil with the lowest content of clay. The incorporation of labeled N in organic forms and its retention in these forms was not directly related to the content of clay in the soils, presumably because the 2 soils with the high content of clay had a relatively high content of available unlabeled soil-N which was used for synthesis of metabolic material. The proportionate retention of labeled C for a given soil was largely independent of the size of the amendments, whereas the proportionate amount of labeled N incorporated into organic forms increased in the clay-rich soils with increasing size of amendments. Of the total labeled C remaining in the soils after the 1st mo. of incubation 50-70% was acid hydrolyzable, compared to 80-100% of the total remaining labeled organic N. This relationship held throughout the incubation and was independent of the size of the amendment and of the temperature of incubation. During the 2nd, 3rd and 4th yr of incubation the half-life of labeled amino acid-N in the soils was longer than the half-life of labeled amino acid-C, presumably due to immobilization reactions. Some of the labeled organic N when mineralized was re-incorporated into organic compounds containing increasing proportions of native soil-C, whereas labeled C when mineralized as CO2 disappeared from the soils. The amount of labeled amino acid-C, formed during decomposition of the labeled cellulose, and retained in the soil, was proportional to the clay content. This amount was .apprx. 3-times as large in the soil with the highest content of clay as in the soil with the lowest content. This difference between the soils was established during the first 10 days of incubation when biological activity was most intense, and it held throughout the 4 yr of the incubation; proportionally it was independent of the amount of cellulose added and the temperature. The labeled amino acid-N content was not directly related to the amount of clay in the soil, presumably because more unlabeled soil-N was available for synthesis of metabolic material in the 2 clay-rich soils than in those soils with less clay. The effect of clay in increasing the content of organic matter in soil is possibly caused by newly synthesized matter, extracellular metabolites, as well as cellular material, forming biostable complexes and aggregates with clay. The higher the concentration of clay the more readily the interactions occurs.