14C-labeled substrates (glucose, hemicellulose, cellulose, maize straw and barley straw) were incubated in 4 soils with clay contents of 6, 12, 16 and 34%. After 2 yr an average of 20% of the labeled C remained in the soils; 10% of this residual C was in biomass as determined by fumigation with CHCl3. Air-drying, C addition (unlabeled glucose), heating (80.degree. C), and grinding of the soils accelerated the evolution of labeled CO2. Grinding and heating had the largest effect, increasing CO2 evolution during the first 10 days by a factor of 15 to 22 relative to untreated soil. Air-drying had the least effect; it increased the CO2 evolution 7-9 times. The accelerating effect was still measurable during the 3rd month of incubation when the CO2 evolution was 1.2 to 1.9 times that from untreated soil. The treatments also affected the labeled biomass; air-drying had the least effect and grinding the most. Three months after these 2 treatments the biomass was 3/4 and 1/4, respectively, of the amount at the start. On the average the treatments in all 4 soils had the greatest affect on humified material originating from glucose, hemicellulose and cellulose; the least effect was on material originating from straw. The addition of unlabeled glucose accelerated the evolution of labeled CO2-C in all 4 soils. The size of the effect on CO2 evolution and on the biomass was similar to that of air-drying. Grinding killed a larger percentage of the biomass in the sandy soil than in the soils with a high content of clay. The effect of the other treatments was largely the same in all 4 soils. The effect of the treatments towards the native biomass and humic matter was largely parallel to that on the labeled biomass. The observations are consistent with the view that the biomass as determined by fumigation with CHCl3 mainly consists of dormant organisms. CO2 production, the biological activity, was related to the amount of available organic material and not the size of the biomass.