Specific leukocyte/endothelial interactions are critical for immunity and inflammation, yet the molecular details of this interaction interface remain poorly understood. Thus, we investigated, with confocal microscopy, the distribution dynamics of the central adhesion molecules ICAM-1 and LFA-1 in this context. Monolayers of activated HUVECs stained with fluorescent anti-ICAM-1 Fabs or Chinese hamster ovary-K1 cells expressing ICAM-1-green fluorescent protein were allowed to bind LFA-1-bearing monocytes, neutrophils, or K562 LFA-1 transfectants. ICAM-1 was rapidly relocalized to newly formed microvilli-like membrane projections in response to binding LFA-1 on leukocytes. These ICAM-1-enriched projections encircled the leukocytes extending up their sides and clustered LFA-1 underneath into linear tracks. Projections formed independently of VCAM-1/very late Ag 4 interactions, shear, and proactive contributions from the LFA-1-bearing cells. In the ICAM-1-bearing endothelial cells, projections were enriched in actin but not microtubules, required intracellular calcium, and intact microfilament and microtubule cytoskeletons and were independent of Rho/Rho kinase signaling. Disruption of these projections with cytochalasin D, colchicine, or BAPTA-AM had no affect on firm adhesion. These data show that in response to LFA-1 engagement the endothelium proactively forms an ICAM-1-enriched cup-like structure that surrounds adherent leukocytes but is not important for firm adhesion. This finding leaves open a possible role in leukocyte transendothelial migration, which would be consistent with the geometry and kinetics of formation of the cup-like structure.