Intercellular adhesion molecule-5 (ICAM-5, telencephalin) is a member of the immunoglobulin superfamily expressed on telencephalic neurons, and serves as a ligand for the leukocyte integrin CD11 a/CD18. We studied here the binding site in ICAM-5 for CD11a/CD18. Protein constructs containing the first immunoglobulin domain of ICAM-5 were able to support CD11a/CD18 interaction, while deletion of the first domain abolished binding. Monoclonal antibodies reacting with the first domain of ICAM-5 also completely blocked the interaction. The soluble first domain of ICAM-5 inhibited the binding of T cells to immobilized ICAM-5 at concentrations of 50 nM and higher. Interestingly, the sixth domain of ICAM-5 was also able to support leukocyte binding, but this binding activity may not involve leukocyte integrins. To test the involvement of ICAM-5 in leukocyte-neuron interactions, an assay using human T cells binding to rat hippocampal neurons was established. This binding was blocked by monoclonal antibodies against CD11a/CD18 and ICAM-5. Thus ICAM-5 may act as a major adhesion molecule for leukocyte binding to neurons in the central nervous system.