The chemokine receptor CXCR5/BLR1 is expressed on mature B cells and a small subpopulation of T memory cells. Using gene targeting in mice, CXCR5 could be identified as the first chemokine receptor involved in homeostatic trafficking of lymphocytes to and within secondary lymphoid organs. CXCR5-deficient mice lack B cell follicles in spleen andPeyer's patches. As the chemokine BLC/BCA-1, a ligand for CXCR5, is expressed on follicular stroma cells, both molecules represent a receptor-ligand pair that mediates the formation of follicles in lymphoid organs. In addition, CXCR5-deficient mice lack defined lymph nodes, suggesting that this chemokine receptor also plays a pivotal role in lymphoid organ development.