Bone morphogenetic protein (BMP)-2 has a critical role in bone formation and regeneration. Therefore, the ability to immobilize this molecule in certain matrices may be crucial in bone tissue engineering. Using carbodiimide chemistry, BMP-2 was directly immobilized on silk fibroin films. Whereas human bone marrow stromal cells cultured on unmodified silk fibroin films in the presence of osteogenic stimulants exhibited little if any osteogenesis, the same cells cultured on BMP-2 decorated films in the presence of osteogenic stimulants differentiated into an osteoblastic lineage as assessed by their significantly elevated alkaline phosphatase activity, calcium deposition, and higher transcript levels of collagen type I, bone sialoprotein, osteopontin, osteocalcin, BMP-2, and cbfal. Using cell culture inserts, it was demonstrated that differentiation was induced by the immobilized protein and not by protein released into the culture medium. Comparison with a similar amount of medium-supplemented BMP-2, where no additional protein was added with medium changes, showed that delivery of BMP-2 immobilized on the biomaterial surface was more efficient than soluble delivery. The results illustrate that BMP-2 covalently coupled on silk biomaterial matrices retains biological function in vitro based on the induction of osteogenic markers in seeded bone marrow stromal cells. © 2004 Wiley Periodicals, Inc.