Background: Spinal fusion is indicated in the surgical management of various spinal disorders. To ensure stabile fusion, bone graft materials are essential. Traditionally allo- or autograft has been used, but both are associated with limitations. Synthetic bone graft materials that reassemble today's gold standard are highly desired. Uninstrumented posterolateral fusion (PLF) is one of the most challenging bone graft procedures because of large graft size and lack of external stability. P15 is a synthetic 15 amino acid peptide sequence, identical to the biding site for alpha2-beta1 integrin on the surface of bone forming cells. The binding initiates natural intra- and extracellular signalling pathways, inducing production of growth factors, bone morphogenic proteins and cytokines. P15 peptide has previously shown to improve osteoinductive properties when coated on graft materials. Purpose: In this study, we compared fusion rates between silk fibre enhanced anorganic bovine derived hydroxyapatite matrix (ABM) with and without P15 peptide coating in uninstrumented PLF in a preclinical setting. Study design: Randomised prospective study in sheep. Method/materials: Twelve Tex/got sheep underwent open two level uninstrumented PLF at level L2/L3 and L4/L5. Levels were randomised to receive silk fibre enhanced ABM graft with or graft without P15 coating. The sheep were sacrificed after 4.5 months. Levels were harvested and evaluated with Micro-CT 50 scanner and qualitative histology. Fusion rates were assessed with 2D sections and 3D reconstruction images and fusion was defined as intertransverse bridging. Results: Spinal fusion was found in 72 % of levels receiving silk fibre enhanced ABM/P15 graft material. In levels with silk fibre enhanced AMB fusion rate was 41 %. These findings are significant different (P