The role of mononuclear leukocytes for the migration of smooth muscle cells (SMCs) during intimal thickening was investigated in the rabbit model of electrically stimulated carotid artery. The approach was to inhibit leukocyte entry into the arterial intima with antibodies against the adhesion molecules very late activation antigen-4 (VLA-4) and CD11/CD18. In electrically stimulated control rabbits treated either with saline or a nonspecific antibody, all types of granulocytes, monocytes, and lymphocytes migrated across an intact endothelium into the acellular subendothelial space, followed by the movement of SMCs from the media into the intima within 36 hours of applying electrical current. Treatment of the rabbits with monoclonal antibody (mAb) HP1/2 directed toward the alpha 4 subunit (CD49d) of VLA-4 inhibited mononuclear leukocyte invasion (consisting of monocytes and lymphocytes) by approximately 70% compared with the IgG-treated control rabbits and completely abolished the minimal influx of basophils and eosinophils after 36 hours. Neutrophil infiltration, however, remained unaffected by anti-VLA-alpha 4 treatment. Under these conditions, SMC migration across the internal elastic lamina was reduced by 50%. The use of mAb HP1/2 together with mAb 60.3 (directed to the beta 2 chain of CD11/CD18) completely abolished the influx of monocytes, lymphocytes, and all types of granulocytes into the arterial intima. This complete blockade of leukocyte infiltration resulted in a 70% reduction of intimal SMC accumulation. Together with our previous findings excluding neutrophils as stimulators of SMC migration, the present results indicate that mononuclear leukocyte promote lesion development by stimulating SMC migration.