Follicular dendritic cells (FDC) store native antigen for long periods In tymphoid follicles and so provide a source ofcontinued stimulation for specific B cells. The expression of MHC class II by FDC suggested they might act as antigen-presenting cells for MHC class ll-restricted T cells. We show here, however, that the MHC class II molecules found on their surface are not synthesized by the FDC but are picked up from surrounding B cells In germinal centres. Although FDC by themselves cannot present native antigen to T cells, acquired MHC class ll-peptlde complexes can be recognized by T cells. The true physiological role of FDC seems to be as long-term antigen depots. We demonstrate that antigen localized onto FDC In vivo can be retrieved by antigen-specific B cells, which In turn process and present It to T cells. Thesepresentation pathways are likely to be crucial In both the maintenance of long-term Immune responses and the continued survival of memory cells.