CD40 was originally described as a functionally significant B cell surface molecule. However, CD40 is also expressed on monocytes, dendritic cells, epithelial cells, and basophils. We now report that synovial membrane (SM) or dermal fibroblasts also express cell surface CD40 in vitro. Fibroblast CD40 expression declines with increasing time in culture and recombinant interferon‐γ (rINF‐γ) induces fibroblast CD40 up‐regulation. This effect of rINF‐γ is augmented by recombinant interleukinlα or recombinant tumor necrosis factor‐α. CD40 expression on fibroblasts is functionally significant because CD40L‐CD40 interactions induce SM fibroblast CD54 (intercellular adhesion molecule‐1) and CD106 (vascular cell adhesion molecule‐1) up‐regulation. Moreover, ligation of CD40 augments IL‐6 production by SM fibroblasts and induces fibroblasts to proliferate. In addition, rINF‐γ enhances the effect of CD40L‐CD40 interactions on fibroblast proliferation. Taken together, these studies show that fibroblasts can express CD40, cytokines can regulate fibroblast CD40 expression, and CD40 ligation induces fibroblast activation and proliferation. J. Leukoc. Biol. 58: 209–216; 1995.