We describe the phenotype of gene-targeted mice lacking the putative chemokine receptor BLR1. In normal mice, this receptor is expressed on mature B cells and a subpopulation of T helper cells. Blr1 mutant mice tack inguinal lymph nodes and possess no or only a few phenotypically abnormal Peyer's patches. The migration of lymphocytes into splenic follicles is severely impaired, resulting in morphologically altered primary lymphoid follicles. Furthermore, activated B cells fail to migrate from the T cell-rich zone into B cell follicles of the spleen, and despite high numbers of germinal center founder cells, no functional germinal centers develop in this organ. Our results identify the putative chemokine receptor BLR1 as the first G protein-coupled receptor involved in B cell migration and localization of these cells within specific anatomic compartments.