The critical requirement for matrix-associated bone morphogenetic proteins (BMPs) during induction of bone formation in vivo has long been recognized. However, the role of extracellular matrix (ECM) physisorbed BMPs in inducing the differentiation of resident mesenchymal stem cells into osteoblasts has been ill-defined. We therefore used BMP-responsive C2C12s to study the biological activity of collagen type I physisorbed BMP-2. Fibrillar collagen type I scaffolds were loaded with 75 ng BMP-2/μg collagen. Under cell culture conditions, 40% of loaded ¹²⁵I-labelled BMP-2 was released within 24 h, whereas the remaining BMP-2 was stably physisorbed for >7 days. Using these systems suggested that physisorbed BMP-2 is more active than diffusible BMP-2. Thus, the current clinical practice of immobilizing BMPs on collagen type I scaffolds not only prolongs local delivery of the morphogen but could also enhance biological activity at the cellular level.