A preliminary investigation was conducted on an alternative approach to obtaining measurements of biologically fixed N2 by the 15N dilution technique. This approach was aimed at removing the requirement for a nonfixing reference plant. The organic N in soil was labelled by incorporating 15N-enriched plant material or by immobilizing 15N-enriched NH4+-N. A nonnodulating isoline of soybean (cv. Lee) was grown on two labelled soils in pots in a growth room, and the 15N enrichment of the plants was determined after 16 weeks of growth. The isotopic composition of inorganic N released during incubation of the labelled soils under controlled conditions in the growth room and laboratory was also determined. Close agreement was found between the 15N enrichments of plant material and nitrogen mineralized in unplanted pots maintained in the growth room. These results suggest that for plants grown in pot culture the proportion of N derived from fixation could be determined from the dilution of 15N in the plant relative to the 15N enrichment of the N mineralized. Laboratory studies demonstrated that the 15N enrichment of nitrogen mineralized decreased with time, suggesting that the application of this approach to field assessment of N2 fixation would require uniform labelling of the soil organic nitrogen.