The nature, kinetics and cofactor requirements of leukocyte adhesion to cultured vascular cells have been investigated in vitro, using a model system in which leukocyte suspensions are in continuous motion over the cultured cells. Adhesion is assessed by histological examination or by using 51Cr-labelled leukocytes. Leukocytes adhere preferentially to endothelial cell monolayers rather than to smooth muscle cells, adventitial fibroblasts or serum-coated glass. Arterial and venous endothelium are equally good substrates for leukocyte adhesion, and lymphocytes and granulocytes at the same suspension density adhere to endothelial cells in similar numbers. Adherence is greatest during the first 10 min, and is inversely related to flow rate. Numbers of leukocytes adhering to the cell monolayer are proportional to the initial concentration of leukocytes in suspension, although only 2--10% of the leukocytes adhere. Leukocytes that have adhered to the cell monolayer spread over the cells and then migrate, apparently through intercellular junctions. Adhesion requires Ca2+ or Mg2+ ions, but not plasma protein cofactors. Fewer leukocytes adhere to endothelial cells grown for 48 h in rapidly stirred medium than to cells grown under conventional, static conditions.