Adhesion between osteoblasts and osteoclast precursors is established via an interaction involving intercellular adhesion molecule-1 (ICAM-1) on osteoblasts and leukocyte function-associated antigen-1 (LFA-1) on osteoclast precursors. The latter cells also express ICAM-1, but little is known about the expression over time and its possible role during osteoclastogenesis. In the present Study we investigated the expression of ICAM-1 on both human osteoblast-like cells and osteoclast precursors in a co-culture. The protein expression on osteoclast precursors strongly increased whereas the osteoblast-like cells became ICAM-1 negative. Interestingly, ICAM-1 on osteoclast precursors manifested as Clusters which localized at the baso-lateral membrane. Furthermore, clustered ICAM-1 was associated with F-actin and remained present for several days. Our data Suggest that osteoblastic ICAM-1 is mainly involved in the initial adhesion of osteoclast precursors whereas Clustered ICAM-1 on osteoclast precursors and its association with F-actin suggest an involvement in cell movement at a later stage. (C) 2009 Elsevier Inc. All rights reserved.