The interaction of transforming growth factor β (TGF β) with extracellular matrix macromolecules was examined by using radiolabeled TGF β and various matrix macromolecules immobilized on nitrocellulose. TGF β bound to collagen IV with greater affinity than to other extracellular matrix macromolecules tested. Neither laminin nor fibronectin, both of which bind type IV collagen, interfered with the binding of TGF β to type IV collagen. TGF β₂ competed effectively with TGF β₁ for binding to type IV collagen. The biological effect of TGF β was tested by an assay based on inhibition of proliferation of an osteoblast cell line, MC3T3-E1. The results demonstrated that the effect of TGF β₁ was sustained when cells were grown on type IV collagen compared to cells grown on laminin, collagen type I, and plastic. These results demonstrate that extracellular matrix components may function as an affinity matrix for binding and immobilizing soluble growth and differentiation factors. In view of the demonstrated role of basement membranes in development the present results imply an important function for transforming growth factor β bound to collagen IV in local regulation of cell proliferation and differentiation.