OBJECTIVE To analyze the expression of the CD(40) and its ligand (CD(40)L) on bone marrow stromal cells (BMSC), and investigate interleukin-3 (IL-3), IL-6, Flt3 ligand (FL) and stem cell factor (SCF) production of BMSC after stimulated with CD(40) agonistic monoclonal antibody (5C11) and its role in the regulation of hematopoiesis. METHODS BMSC were freshly isolated from adult bone marrow. The expression of CD(40) on these cells was determined with flow cytometry, the concentrations of IL-3, IL-6, FL and SCF in the supernatant at 24, 48 and 72 hours after BMSC cultured with 5C11 at a dose of 20 micro g/ml were determined by the ELISA assay. After BMSC incubated with 5C11 for 24 hours, the supernatant was collected and cultured with purified cord blood CD(34)(+) cells for 7 days under 37 degrees C in a fully humidified atmosphere supplement with 5% CO(2). Colony formation assay and Annexin V assay were employed to determine the proliferation and apoptosis of the CD(34)(+) cells. RESULTS BMSC expressed CD(40) and the production of IL-6 and FL increased after stimulated with 5C11 while IL-3 and SCF had no change. The supernatant collected from the stimulated BMSC promoted proliferation of CD(34)(+) cells, increased the CFU-GM yields and had anti-apoptosis effects. CONCLUSIONS CD(40) ligandization on BMSC increased the production of IL-6 and FL and promoted the proliferation of CD(34)(+) cells. The couple CD(40)/CD(40)L may be involved in the control of hematopoiesis via modulation of the cytokine network in the bone marrow.