To investigate the effects of transforming growth factor beta1 (TGF-beta1) on dendritic cells (DC).Murine bone marrow cells were cultured with different cytokine combinations to develop immature DC (imDC, GM-CSF only) and TGFbeta-DC (GM-CSF + TGF-beta1), and their responses to lipopolysaccharide (LPS) stimulation were observed. The cell ultrastructure was observed by transmission electron microscopy and their phenotypes were assessed by flow cytometry (FCM). The allogeneic stimulating capacity of DC was assayed by mixed lymphocyte reaction (MLR) with BrdU incorporation. IL-12p70 protein was detected by ELISA and the expressions of Toll-like receptor 4 (TLR4) on DCs were analyzed with semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR).Compared to imDC, the TGFbeta-DC had no significant alterations in ultrastructure after LPS stimulation. The expressions of CD80, CD86 were lower on TGFbeta-DC than on imDC [(4.14 +/- 0.95)% vs (13.90 +/- 7.22)%; (8.60 +/- 0.75)% vs (20.63 +/- 5.03)%, P < 0.05, both]. The TGFbeta-DC kept their immature morphology after LPS stimulation, but the expressions of I-Ab and CD80 were slightly increased. After 96 h MLR, TGFbeta-DC had weaker stimulating capacity than imDC did, especially when DC/T cells ratios were 1:4 and 1:1 (P < 0.05, both). TGFbeta-DC showed impaired IL-12p70 production and down-regulation of TLR4 expression.TGF-beta1 can inhibit the expression of co-stimulatory molecules on DC. The TGFbeta-DC is resistant to maturation stimulus (LPS) and might be linked with TLR4 down-regulation.