Receptor activator of nuclear factor kappa-B ligand (RANKL) expression was examined during the development of mouse fetal peripheral lymphoid organs. A shift in the expression pattern was detected during the transition from lymphoid tissue inducer (LTi) cells to lymphoid tissue organizer (LTo) cells in the lymph node (LN) anlagen but not in the Peyer's patch anlagen. In order to understand the functional impact of these changes in the fetal expression of RANKL, the RANKL function was blocked by a blocking antibody. Excess anti-RANKL antibody was administered to pregnant mice between 13.5 and 16.5 dpc and was found to completely block LN anlagen development, suggesting that RANKL function during this period is critical for LN development. In addition, small amounts of anti-RANKL antibodies were injected directly into the amniotic space at 13.5 dpc, resulting in perturbed B-cell follicle formation and high endothelial venule differentiation after birth. These results suggest that RANKL expression on LTi cells during the early phase of LN development is critical for the development LN microarchitecture.