A greenhouse experiment was conducted on three soils with differing cropping and fertilization histories to determine whether unenriched fertilizer N can be used in the same manner as15N-enriched fertilizer to estimate the amount of plant N derived from fertilizer. Estimates using unenriched fertilizer N were compared with estimates using two15N enrichment levels. Use of unenriched fertilizer N led to underestimation of the amount of fertilizer N in the plant material in four of six cases when compared to15N-enriched fertilizer. Standard deviations of the estimates of fertilizer-derived N in plant material were considerably greater when unenriched fertilizer was used.