Follicular dendritic cells (FDC) are unique non‐lymphoid cells found only in lymph follicles. They play a part in the survival, proliferation and differentiation of B cells. To analyse the influence of FDC on B‐lymphocyte proliferation, we isolatedthem from human tonsils on albumin gradients and treated them with mitomycin C to prevent the multiplication of lymphoid cells harboured in their cytoplasmic evaginations. FDC cultured for 12–16 h remained attached to the substrate;non‐adherent cells were carefully eliminated by washing. Purified B cells cultured alone or with contaminant‐cleared FDC were maintained for 2 days in the presence or absence of various stimulants, after which tritiated thymidine uptake by these cells was measured. In the absence of activators, FDC did not induce B‐cell multiplication. B cells cultured in the presence of FDC exhibited increased3H‐TdR uptake when activated with anti‐CD40 MoAb, anti‐immunoglobulin MoAb or transferrin, but notwhen stimulated withStaphylococcus aureusstrain Cowan I (SAC) at a given concentration. In the latter case, B‐cell proliferation clearly decreased. In control cocultures where mitomycin‐C‐treated non‐adherent cells were used instead of FDC inthe presence of the different stimulants, no increase in B‐cell proliferation was observed. The results suggest that, inside the germinal centres, FDC modulation of B‐cell proliferation depends on the activation state of the B cells