We evaluated the relative contribution of ICAM-1 and ICAM-2, known ligands on endothelium for LFA-1 and Mac-1, in spontaneous neutrophil (PMN) transendothelial migration (TEM) across IL-1-activated HUVEC monolayers or TEM induced by C5a or IL-8 across unstimulated HUVEC grown on polycarbonate filters. Adhesion blocking mAb to ICAM-1 [R6.5 F(ab)2] or ICAM-2 [CBR IC2/2 F(ab)2] tended to inhibit TEM under each condition but, in general, inhibition was significant only with both ICAM-1 and ICAM-2 blockade. mAb to LFA-1 partially inhibited migration to C5a or IL-8 across unstimulated HUVEC and inhibition was not altered by additional treatment of HUVEC with mAbs to ICAM-1 and -2. In contrast, with IL-1 HUVEC, mAb to ICAM-1 significantly inhibited this LFA-1-independent TEM. mAb to Mac-1 alone partially inhibited TEM and, when combined with mAb to LFA-1, migration was almost completely blocked with all TEM conditions tested. The contribution of alternate ligands for Mac-1 in mediating Mac-1-dependent but ICAM-1/-2-independent C5a-induced TEM was examined using anti-LFA-1-treated PMN and anti-ICAM-treated resting HUVEC. Addition of RGD peptides, fibronectin, fibrinogen, heparins, collagens alone or in combination, even to heparinase-treated HUVEC, did not inhibit this Mac-1-mediated PMN TEM. The results indicate that: (1) LFA-1 mediates PMN TEM primarily by interaction with ICAM-1 and ICAM-2; (2) ICAM-2 may function in concert with ICAM-1 in this role, especially on unstimulated endothelium, and (3) Mac-1 on PMN also plays a major role in TEM and can utilize yet to be identified ligands distinct from ICAM-1 or -2, especially on unstimulated endothelium.