Mouse CD275 (B7-H2) is a member of the growing B7 family of immune costimulatory proteins. CD275 has been identified as the ligand for ICOS, a member of the CD28 family of co-stimulatory receptors. Mouse CD275 is expressed on resting B cells and at low levels on monocytes. The CD275/ICOS interaction appears to play roles in T cell dependent B cell activation and Th differentiation.