Cell-cell and cell-matrix adhesive interactions mediated by integrins play crucial roles in leukocyte migration to inflamed tissues, and also in cell migration during embryogenesis. Much remains to be learned about the molecular mechanisms of regulation of adhesion mediated by integrins. Recently we found that steel factor and c-kit induce adhesion to fibronectin by VLA-5 in mast cells. Activation of adhesiveness is transient, and occurs at concentrations of steel factor 100-fold lower than required for growth stimulation. This suggests that regulation of adhesion is an important biological function of steel factor and c-kit. Other receptor tyrosine kinases such as the PDGF receptor can substitute for c-kit. Signaling through receptor tyrosine kinases may offer a general mechanism for the regulation of integrin avidity.