The ability of integrins to mediate cell attachment to extracellular matrices and to blood proteins is regulated from inside the cell. Increased ligand-binding activity of integrins is critical for platelet aggregation upon blood clotting and for leukocyte extravasation to inflamed tissues. Decreased adhesion is thought to promote tumor cell invasion. R-Ras, a small intracellular GTPase, regulates the binding of integrins to their ligands outside the cell. Here we show that the Eph receptor tyrosine kinase, EphB2, can control integrin activity through R-Ras. Cells in which EphB2 is activated become poorly adherent to substrates coated with integrin ligands, and a tyrosine residue in the R-Ras effector domain is phosphorylated. The R-Ras phosphorylation and loss of cell adhesion are causally related, because forced expression of an R-Ras variant resistant to phosphorylation at the critical site made cells unresponsive to the anti-adhesive effect of EphB2. This is an unusual regulatory pathway among the small GTPases. Reduced adhesiveness induced through the Eph/R-Ras pathway may explain the repulsive effect of the Eph receptors in axonal pathfinding and may facilitate tumor cell invasion and angiogenesis.