The monoclonal antibody 5B8 reacts with mouse osteoclast-associated receptor (OSCAR), which is a 265 amino acid type 1 transmembrane protein belonging to the leukocyte receptor complex protein family that play critical roles in the regulation of both innate and adaptive immune responses. In mice, OSCAR is a RANKL-inducible gene which- is expressed during the later stages of pre-osteoclast maturation and is shown to be a potent FcRγ-associated costimulatory receptor in vitro. OSCAR promotes osteoclastogenesis in vivo, and OSCAR binding to its collagen ligand led to signaling that- is involved in functional differentiation of leukocytes. In recent studies human OSCAR has been shown to play multiple roles in the immune system including activation and maturation of immune cells, prevention of apoptosis and enhancement of pro-inflammatory circuits. The latter by- inducing intracellular calcium release resulting in sustained IL-8 and chemokine expression leading to chemotaxis of Th2 effector and regulatory T-cells. In a- murine model using calvarias bone explants, the synovial fluid of osteoarthritis patients resulted in increased OSCAR mRNA expression suggesting that OSCAR might contribute to in the degenerative process. Moreover, in a recent study- OSCAR was found to be- induced in monocytes of RA patients, facilitating their differentiation into osteoclasts and bone resorption. The monoclonal antibody, 5B8, can be used for analysis of mouse OSCAR in flow-cytometry as well as functional studies for osteoclastogenesis.