Ag-bearing follicular dendritic cells (FDC) are found throughout secondary lymphoid tissues in close association with rapidly proliferating germinal center B lymphocytes. We reasoned that FDC might provide costimulatory signals that would enhance the ability of Ag to stimulate B cell proliferation in the germinal centers. To test this, FDC were cultured with B cells activated by a slg-dependent (goat anti-mouse mu conjugated to dextran (anti-mu-dex)) or -independent (LPS) pathway and their proliferation was measured by using [3H]thymidine incorporation. The addition of FDC markedly augmented B cell proliferation in a dose-dependent fashion. Depletion of FDC from cultures abrogated the increased proliferation. Addition of highly purified FDC obtained from cell sorting resulted in B cell costimulation, whereas addition of other sorted cells was without effect. The FDC accessory activity was apparent over the entire culture period and over a wide range of either polyclonal B cell activator. When B cells and activators were cultured in the absence of FDC, only about one fourth of the cells remained viable after 3 days. In contrast, virtually all cells in cultures containing FDC, B cells, and activator were viable. Cultures containing FDC and B cells from nude mice proliferated normally in the presence of anti-mu-dex plus rIL-4, implying that IL-4 provides adequate T cell help in this system. The costimulatory activity of the FDC could not replace either the anti-mu-dex or IL-4 in this system and was not MHC restricted. These data support the concept that FDC not only provide Ag but also facilitate B cell proliferation by means of other costimulatory interactions that contribute to make the microenvironment in the germinal center favorable for B cells to proliferate.