Microbial biomass is determined from the excess of extractable organic N released from fumigated soil samples. In the presence of relatively high contents of labeled mineral N, small differences in organic N and N may not be detectable. Two approaches were tested to determine organic N content and its N enrichment in the presence of considerably greater concentrations of labeled mineral N: (i) Removal of mineral N from mixed solutions of alanine and NH4or NO3by reduction and boiling under alkaline conditions, prior to Kjeldahl digestion. (ii) Including mineral N in Kjeldahl N analysis, by reduction under acidic conditions prior to digestion and calculating organic N and N content by subtracting mineral N and N. The removal of mineral N was either incomplete‐ particularly regarding labeled mineral N, or partly destroyed organic N as well. When mineral N was included in the digest, the recovery of N and15N was sufficiently accurate to obtain good results of organic N and15N by subtracting the known quantities of mineral N, even when organic N was only 20% of the total Kjeldahl N. This procedure was used to determine the flush of Kjeldahl N in fumigated soils that were incubated with labeled mineral N and cellulose.