Sunflower is a good candidate for developing plant protein sources for human nutrition but suffers from a lack of data on the bioavailability of protein and amino acids. The aim of our study was to produce a sunflower isolate intrinsically labelled with 15N to precisely measure its digestibility in humans. Isotopic nitrogen enrichment of sunflower seeds was carried out using 15N fertilizer. The harvested seeds (30 kg) were deoiled by cold pressing and hexane extraction and were desolventized at low temperature (<50°C). A 15 kg batch of sunflower meal was obtained and used to prepare protein isolates by solubilization and ultrafiltration. Biscuits were prepared from the isolate and were incorporated in diets of 7 healthy volunteers. Ileal digestibility was determined by measuring nitrogen and 15N enrichments in the ileal contents by EA-IRMS. The digestibility of the amino acids was determined by measuring their 15N enrichment, by GC-C-IRMS and by quantifying the amino acids by UHPLC. The DIAAS (Digestible Indispensable Amino Acid Score) was thus calculated. The 15N enrichment and the protein purity of the isolate were 0.81 atom percent and 79.1%, respectively. The measured ileal digestibility was 86.0±4.0% of the nitrogen ingested. Ileal amino acid digestibility was 87.0±6.1% in average and varied from 64.4% for glycine to 91.0% for glutamate and glutamine. Digestibility of lysine was 84.4%, resulting in a 0.9 DIAAS value for lysine, a value close to our data previously obtained in rats. Sunflower protein despite a lysine deficiency can be extracted into a protein isolate.