CXCR5 (also known as BLR1) is a GPCR that binds selectively to the chemokine CXCL13 (also known as BLC and BCA-1) to mediate immune system development and function (Förster et al., 1996; Gunn et al., 1998). CXCR5 cooperates with CCR7 to determine functional organization of lymphoid organs such as lymph nodes and Peyer’s patches (Ohl et al., 2003). In addition, CXCR5 mediates migration of B cells into the follicles of the splenic white pulp, thus permitting antigen deposition on follicular dendritic cells (Förster et al., 1996; Cinamon et al., 2008). Patients with rheumatoid arthritis (RA) displayed upregulated expression of CXCR5 to a greater extent than other chemokine receptors, and CXCR5-null mice subjected to antigen-induced arthritis display reduced joint destruction (Schmutz et al, 2004; Wengner et al, 2007). Therefore, CXCR5 represents a potential target for treatment of rheumatoid arthritis. Millipore’s cloned human CXCR5-expressing cell line is made in the Chem-10 host, which supports high levels of recombinant CXCR5 expression on the cell surface and contains optimal levels of promiscuous G protein to couple the receptor to the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for agonists and antagonists at CXCR5.