Steel (SI) and white spotting (W) loci encode steel factor (c-kit ligand) and the c-kit tyrosine kinase receptor, respectively. Mutations at these loci affect migration and differentiation of primordial germ cells, neural crest-derived melanoblasts, and hematopoietic cells. In these processes, cell adhesion molecules are hypothesized to be crucial. We have examined the role of steel factor and c-kit in cell-extracellular matrix adhesion using bone marrow-derived mast cells as a model system. Steel factor stimulates mast cells to bind to fibronectin and, to a lesser extent, to vitronectin, whereas interleukin-3 and interleukin-4, which are also mast cell growth factors, do not. Activation of adhesiveness is transient, occurs at concentrations of steel factor 100-fold lower than required for growth stimulation, and requires the integrin VLA-5. Mast cells from c-kit mutant mice adhere to fibronectin on stimulation with phorbol 12-myristate 13-acetate (PMA), but not on stimulation with steel factor, indicating that stimulation of integrin adhesiveness requires activation of the c-kit protein tyrosine kinase. By contrast, c-kit mutant and wild-type mast cells adhere equally well to COS cells expressing membrane-anchored steel factor, showing that the kinase activity of c-kit is not required for adhesion directly mediated by c-kit. Our findings suggest that regulation of adhesion is an important biologic function of steel factor.