Two soils with relatively high (Soil 1) and low (Soil 2) ammonium fixation capacities were used in thisstudy to extalne the effect of ammonium fixation on the determination of N mineralised from soil ndcrobialbiomass. organism suspellsioll was quantitatively introduced to Soil 1 at various rates. Both fumigation-incubation (FI) and fumigation-ext raction (FE ) met hods were used to t reat t he soil. The amount of ffeedNH4+-N increased with increasing rate of organism-N addition. A close correlation was found between theamoun of fixed aznmonium and the rate of organism-N addition. The net increso of fixed NH4+-N wereequivalent to 38% and 12% of the added organism-N for FI and FE treatments, respectively in this specificsoil. To provide isotopic evidence, 15N-labelled organism-N was added to Soils 1 and 2 at 121.4 mg N kg-1.In FI treatment, 22 and 3 mg N kg-1 of labelled N were found in the fraction of fixed NH4+-N in Soils 1 and2 respectively; while in FE treatment, 9 mg N kg-1 of labelled N was found in the fraction of fixed NH4+-Nin Soil 1 only. There was no labelled N in the fraction of fixed NH4+-N in Soil 2. In all of the unfumigated(check) soils, there was little or no labelled N in the fixed fractions, probably because the organism-N addedwas easily mineralized and nitrified. A mean of 0.64 for KN value, the fraction of N ndneralized in the killedmicrobial biomass, was obtained with inclusion of the net increase of fixed NH4+-N. The corresponding valuecalculated with exclusion of the net increase of fixed NH4+-N was 0.46. It was concluded that ammniumfixation was a problem in determination of KN, particularly for soils with a high ammonium fixation capacity.Results also showed that microbial biomass N measurement by FE method was less affected by ammoniumprocess than that by FI method. Two soils with relatively high (Soil 1) and low (Soil 2) ammonium fixation capacities were used in this study to extalne the effect of ammonium fixation on the determination of N mineralised from soil ndcrobial biomass. Organism suspensions were quantitatively introduced to Soil 1 at various rates . Both fumigation-incubation (FI) and fumigation-ext raction (FE) met hods were used to reat he he soil. The amount of ffeedNH4 + -N increased with increasing rate of organism-N addition. A close correlation was found between the amoun of fixed aznmonium and the rate of organism-N addition. The net increso of fixed NH4 + -N wereequivalent to 38% and 12% of the added organism-N for FI and FE treatments, respectively in this specific soil. To provide isotopic evidence, 15N -labelled organism-N was added to Soils 1 and 2 at 121.4 mg N kg-1. In FI treatment, 22 and 3 mg N kg-1 of labeled N were found in the fraction of fixed NH4 + -N in Soils 1 and 2 while in FE treatment, 9 mg N kg-1 of label led was found in the fraction of fixed NH4 + -Nin Soil 1 only. There was no labelled N in the fraction of fixed NH4 + -N in Soil 2. In all of the unfumigated (check) soils, there was little or no labelled N in the fixed fractions, probably because the organism-N added was easily mineralized and nitrified. A mean of 0.64 for KN value, the fraction of N ndneralized in the killed microbial biomass, was obtained with the inclusion of the net increase of fixed NH4 + -N. The corresponding was calculated with exclusion of the net increase of fixed NH4 + -N was 0.46. It was final that ammniumfixation was a problem in determination of KN, particularly for soils with a high ammonium fixation capacity. Results also showed that microbial biomass N measurement by FE method was less affected by ammoniumprocess than that by FI method.