An experiment was done to see if changes in total soil microbial biomass N and ¹⁵N-labelled biomass N could be measured by fumigation-extraction during the 5–20 day period following addition of ¹⁵N-labelled wheat straw to soil. Nitrogen-15 labelled wheat straw was added to a clay-loam soil at a rate equivalent to 76μg Ng⁻¹ soil, with or without an addition of 50μg, Ng⁻¹ soil as unlabelled NH₄NO₃. Measurements were made 5, 12 and 20 days later. Non-amended soil was incubated similarly. Total soil microbial biomass N (biomass N) remained constant throughout the 20 day incubation in the unamended soil. Addition of straw alone increased biomass N from about 46 (in the unamended soil) to about 80 μg Ng⁻¹ soil by day 5, and it remained at this level thereafter. Inorganic N addition caused a further increase in biomass N of about 20 μg N g⁻¹ soil by day 5, it then slowly declined until day 20. About 30% of the N originally in the straw was held in the biomass in the soil incubated with straw alone by day 5 and about 25% when N was added with the straw. These percentages hardly changed upon further incubations, and were equivalent to 23 and 19 μg ¹⁵N-labelled biomass N respectively, meaned over the three sampling times. Thus a relatively large addition of inorganic N with the straw (50 μg N g⁻¹ soil) only slightly decreased the consumption of straw-N by the biomass and did not prolong the complete consumption of the straw-N. It was concluded that fumigation-extraction could reliably measure changes in biomass N and ¹⁵N-labelled biomass N shortly (i.e. at 5 days) after addition of ¹⁵N-labelled straw to soil. Most of the inorganic N initially added with the straw could not be measured 20 days later. While this loss could have been due to denitrification, it seems unlikely under the aerobic incubation conditions used. Other factors may have operated.