CD137, a member of the TNF receptor family, and its ligand are expressed on T lymphocytes and antigen-presenting cells (APC), respectively. During interaction with APC, T lymphocytes receive a potent, costimulatory signal through CD137. Reverse signaling has been demonstrated for the CD137 ligand, which causes activation in monocytes. Here we show that B lymphocytes also receive costimulatory signals through the CD137 ligand. Immobilized CD137 augmented proliferation of preactivated B lymphocytes up to fivefold and immunoglobulin synthesis, up to threefold. CD137 had no effect on resting cells. Further, we show that CD137 is expressed in vivo by follicular dendritic cells (FDC) in germinal centers. Germinal centers form during humoral immune responses and are essential for B lymphocyte affinity maturation. These data imply that, similar to the CD40 receptor/ligand system, which mediates T lymphocyte help to B lymphocytes after the first antigen encounter, the CD137 receptor/ligand system may mediate costimulation of B lymphocytes by FDC during affinity maturation.