Antegren (Natalizumab) is a humanized monoclonal antibody directed against alpha 4-integrin. This antibody binds to the alpha 4 subunit of alpha 4 beta 1-integrin (VLA-4) and alpha 4 beta 7-integrin on leukocytes and blocks the interaction of these integrins with their ligands (VCAM and MadCAM). Disruption of these cell adhesion molecule interactions inhibits the migration of leukocytes through the blood-brain barrier and the trafficking of leukocytes through the extracellular matrix. In clinical studies, short-term treatment of patients with multiple sclerosis (MS) with antegren resulted in a clear reduction of the number of new active brain lesions on magnetic resonance imaging (MRI). Antegren was safe and well tolerated. Two large phase III studies will help to determine the long-term effect of antegren on MRI and clinical outcome in relapsing-remitting MS patients.