A novel class of dehydro-β-proline-containing peptidomi-metics, designed to be effective as α 4 β 1 integrin ligands, has been developed on the basis of the fundamental requirements for the interactions of these transmembrane receptors with bioactive ligands. Dehydro-β-proline ring has been synthesized through an original pathway, involving ring closing metathesis of a diallylamino derivative. The synthesized products showed to be effective and selective as α 4 β 1 integrin antagonists and displayed IC 50 values in the nanomolar range in cell adhesion inhibition assays and in VCAM-1-induced phosphorylation of extracellular-signal-regulated kinases. Significant activity was observed also toward the homologous integrin α 4 β 7 , while they did not display any activity toward selected members of β 1 , β 2 , and β 3 families. A strong dependence on the stereochemistry of the heterocyclic central core could be observed. The great importance of α 4 β 1 integrin in chronic inflammatory and autoimmune diseases suggests a possible exploitation of these ligands as lead compounds for therapeutic tools development.