The present invention provides methods and compositions for modulating transendothelial migration (TEM) of leukocytes. In particular, inhibition of TEM can provide a potent therapeutic approach to treating inflammatory conditions. The invention specifically relates to the discovery that CD99 mediates TEM, because blocking CD99 on either endothelial cells or monocytes bloks migration 80-90%. In conjunction with PECAM inhibitors, TEM blockade approaches 100%. CD99 is involved in a step in TEM that is distal to the step controlled by PECAM.