LFA-1 (CD11a/CD18) mediates leukocyte adhesion by binding to one of its ligands: ICAM-1, ICAM-2 or ICAM-3. Here, we investigated whether stimuli known to induce adhesion to ICAM-1 were also capable of inducing LFA-1-mediated adhesion of T lymphocytes to ICAM-2 and -3 transfectants. We observed that phorbol 12-myristate 13-acetate, Mn2+, cross-linking of CD3 or activating antibodies against LFA-1 enhanced LFA-1-mediated T cell adhesion to ICAM-2 and -3, although to a lesser extent than to ICAM-1. These results indicate that, similar to what has been reported for adhesion to ICAM-1, activation of LFA-1 is also required for adhesion to ICAM-2 and -3. Furthermore, the results suggest that ICAM-1 is the major ligand for LFA-1 on activated T lymphocytes. Interestingly, we observed that in contrast to activating antibodies against CD18, activating antibodies against CD11a were incapable of inducing adhesion of LFA-1 to all three ligands. The antibody MEM-83 stimulated binding to ICAM-1, while at the same time inhibiting the interaction of LFA-1 with ICA8M-2 and -3. The antibody NKI-L16 selectively induced adhesion to ICAM-1 and -2, but not to ICAM-3. Our results suggest that different conformations of LFA-1 are required to support adhesion to ICAM-1, -2 or -3, and that ligands may bind on different sites of the LFA-1 molecule.