Wheat plants were grown for 14 and 25 days in attapulgite under axenic conditions or reinoculated with a soil suspension. After 14 days growth in a 14CO 2 atmosphere, fumigation with chloroform for 20 h released 14C-material from both axenic and reinoculated soil-root samples. The Δ 14C edta (the difference between 14C extracted with saline EDTA from fumigated and unfumigated soil samples) at 14 days was larger by a factor of 1.6–2.4 in the reinoculated soil-root samples than in the axenic samples. After 25 days growth, there was no measurable Δ 14C edta for the axenic culture and a small but positive value for the reinoculated culture. In situ labelling of the soil microbial biomass with 14C-glucose in Roseworthy and Northfield soils and attapulgite showed that the Δc edta obtained by extraction was a fraction of the microbial biomass 14C as measured by the chloroform fumigation-incubation method. The proportions of biomass 14C extracted were 0.26 for attapulgite. 0.39 for Roseworthy soil. and 0.43 for Northfield soil. An amount of 14C-labelled material, equal to the Δ 14C edta was recovered quantitatively from ion exchange resins. Between 17 and 33% of the material was cationic, 34–42% was neutral and 13–43% was anionic. The EDTA-soluble organic material, released by CHCl 3 fumigation, could also be characterized by UV-VIS spectrophotometry, ninhydrin and anthrone assays. For a clay soil, this material contained 31 μg amino acid Ng −1 soil and 2 40μg carbohydrates as glucose equivalents g −1 soil after 7 days incubation. The differences in the amounts extracted from fumigated and unfumigated samples were consistently highest for the clay-rich soil, and followed the expected pattern of microbial biomass turnover between 7 and 20 days incubation in the different soils.