CXCR5, the receptor for B lymphocyte chemoattractant (BLC), is required for normal development of Peyer's patches, inguinal lymph nodes, and splenic follicles. To test the in vivo activity of BLC in isolation of other lymphoid organizers, transgenic mice were generated expressing BLC in the pancreatic islets. In addition to attracting B cells, BLC expression led to development of lymph node-like structures that contained B and T cell zones, high endothelial venules, stromal cells, and the chemokine SLC. Development of these features was strongly dependent on B lymphocytes and on lymphotoxin alpha1beta2 and could be reversed by blocking lymphotoxin alpha1beta2. These findings establish that BLC is sufficient to activate a pathway of events leading to formation of organized lymphoid tissue.