[en] In order to determine the N₂-fixing capabilities of several soybean mutant lines under different N rates and inoculant types, two different field experiments were conducted at Antalya and Adana in 1989. In experiment I mutant soybean lines C_k, C₂, C_g, A_k, A₈, A₉ were investigated at 15 and 30 kg N/ha rates of ammonium sulphate (21 %N), where all seeds were inoculated with ''Azotek'' inoculant. In a second experiment, experiment II same mutant lines were investigated using two inoculant - (Entite and Azotek) where 15 kg Na/ha ammonium sulphate (21 %N) was applied. As reference crop, corn was used in all experiments. ¹⁵N isotope plot with dimensions of 1.0mx1.5m=1.5m² were established in each plot. 2.5% ¹⁵N atom excess for low N rates and 2.0% ¹⁵N atom excess for low N rates and 2.0% ¹⁵N atom excess for high N rates were sprayed to the ¹⁵N isotope plots as 250 ml aqueous solution just after sowing at both locations. At maturity all plants were harvested at the same time. Total N analysis in ground plant samples were done by micro-Kjeldahl procedure and ¹⁵N determinations were made by emission spectrometer. Results showed that increasing the N rate increased the %N dff, %Ndfa and total N fixed by the soybean mutant lines. Also %Ndfa and total N₂ fixed varied between the soybean mutant lines according to the inoculant type, the mutant line C-9 had the highest %Ndfa and total fixed N values, among others