Samples of a sandy soil, which had been incubated for 8 years in the field with [14C]labelled barley straw, were amended with 1, 2, 3 or 4 successive additions of [14C]labelled straw, respectively, applied at intervals of 3 months. The decomposition of the straw was studied over a 4-yr period of laboratory incubation, following the first repeated application, by determination of the total amount of labelled C in the soils and labelled C in the soil amino acids. The overall pattern of decomposition was similar whether the soil was amended with one or with several successive applications. Four years after the first repeated addition of labelled straw the soils were subjected to a number of “stress” treatments: addition of unlabelled glucose, air-drying, oven-drying, grinding and fumigation with vapour of chloroform, respectively. The CO2 that developed during the first 10 days after the treatments, less the evolution from untreated samples, was taken as a measure of the effect of the treatments. The amount of biomass in the soils was calculated from the increase caused by the fumigation with chloroform. In soil incubated undisturbed in the field for 12 yr, biomass accounted for 2.6% of the labelled C in the soil, whereas it was only half this amount in the soil incubated for 8 yr in the field followed by 4 yr in the laboratory. In the soils amended with successive additions of labelled straw, the size of the biomass showed declining values with an increasing number of additions. Biomass thus accounted for 2.6% of the labelled C in the soil amended with one repeated addition, and 1.0% in the soil amended with 4 repeated additions. The increase in the evolution of labelled CO2-C caused by the stress treatments ranged from 0.3 to 1.7% of the labelled C in the soil: air-drying had the least effect, grinding the most. The effect of each treatment declined with an increasing number of successive additions of straw. The ratio between CO2 evolved after grinding and fumigation, respectively, revealed that grinding also exposed non-biomass material to accelerated decomposition. The effects of the stress treatments on the evolution of native CO2-C was on the whole parallel to the effects on the evolution of labelled CO2-C.