CD40 is a 50-kDa protein expressed on B cells, dendritic cells, monocytes and epithelial cells, but the distribution of CD40 expression in humans is not completely known. It binds to a ligand (CD40L) which is expressed essentially on activated T cells. The interaction between CD40 and CD40L plays important roles in immune responses. CD40 expression was investigated on bronchial tissues and human bronchial cell lines using immunohistochemistry, immunofluorescence staining and analysis with a cytometer, respectively. Constitutive CD40 expression, but not that of CD40L, was slightly detectable on normal human bronchial epithelial cells (HBEC) in situ and on an adult lung adenocarcinoma (SKLU1) cell line, while another cell line, a bronchial transformed SV40 cell line (WI26VA4), was negative for CD40. Among the various cytokines tested, only interferon (IFN)-gamma was found to induce CD40 expression on WI26VA4. Tumour necrosis factor (TNF)-alpha was the best cytokine able to up-regulate CD40 in SKLU1 cells. A combination of IFN-gamma and TNF-alpha was slightly more effective than the cytokine alone at up-regulating CD40 expression on both cell lines. We further investigated the functional consequences of CD40 ligation on both cell lines. These bronchial cells expressed CD40, HLADR and CD54 under basal conditions or when stimulated by cytokines. Stimulation through CD40 did not affect cell-surface-antigen expression on either cell line. The production of cytokines such as interleukin (IL)-6 and granulocyte macrophage-colony stimulating factor (GM-CSF) by HBEC has been described. SKLU1 and WI26VA4 cells released IL-6 and GM-CSF spontaneously. Whatever the case, CD40 engagement did not modulate spontaneous or TNF-alpha-induced production of these two cytokines. These data indicate for the first time that normal HBEC express CD40 in situ. Further investigations are required in order to determine the role of CD40 on normal HBEC.