Fractionation of N isotopes occurs in many metabolic reactions which causes tissue proteins to become enriched in 15N while urea (urine) is depleted in 15N relative to the diet. We investigated 15N enrichment of whole plasma and its relationship with feed conversion efficiency (FCE) in growing beef heifers (n 84) offered 2 kg/d of concentrates with grass silage ad libitum. Heifers were on average 299 (sd 48·3) d old and weighed 311 (sd 48·8) kg. Plasma was obtained on day 79 (n 84) of the experiment and from a subset of animals (n 20) on four occasions between days 16 and 79. Silage DM intake (DMI) averaged 4·1 (sd 0·74) kg/d and concentrate DMI was 1·72 kg/d. Mean mid-test live weight was 333 (sd 47·6) kg, daily gain was 0·53 (sd 0·183) kg, FCE (g live-weight gain/g DMI) was 0·09 (sd 0·028) and residual feed intake (RFI) was 0 (sd 0·428). N isotopic fractionation (Δ15N; plasma δ15N − diet δ15N) averaged 3·58 ‰ on day 79 (n 84) and 3·90 ‰ for the subset of heifers. There was no relationship between Δ15N and RFI. Plasma δ15N and Δ15N were related to both FCE (negative) and animal weight (positive) for the whole population, and repeatable for the subset of animals over four time points. These relationships of Δ15N with FCE and animal weight are consistent with the anticipated negative relationship with N-use efficiency. There is potential to use Δ15N to provide rapid, low-cost estimates of FCE in cattle.