Objective To investigate the effects of mannan-binding lectin ( MBL) on the differen-tiation of Th17 cells (T helper cell 17, Th17). Methods CD4+T cells were separated in vitro from fresh human cord blood by MACS ( magnetic-activated cell sorting ) separator. Anti-CD3 monoclonal antibody ( McAb) and anti-CD28 McAb were used to stimulate CD4+T cells with IL-6 and TGF-β1 as inducers. Then, these cells were treated with ( MBL group) or without ( control group) different concentrations of MBL. Percentages of Th17 cells in different groups were detected by fluorescence-activated cell sorting( FACS) after 72 hours of culturing. Quantitative real-time PCR ( Q-PCR) was used to analyze the expres-sion of RORγt ( retinoid-related orphan receptor-γ-t) at mRNA level in both control and MBL groups. En-zyme-linked immunosorbent assay (ELISA) was performed to detect the levels of IL-17A in supernatants of cell culture from different groups. FACS was used to detect the percentages of Th17 cells in MBL-/-and wild type ( WT) mice. Results MBL could significantly reduce the percentage of Th17 cells after 72 hours of culturing as compared with the control group. Moreover, MBL could significantly down-regulate the expres-sion of RORγt at mRNA level and decrease the expression of IL-17A. Results of animal experiments showed that the percentages of CD4+RORγt+Th17 cells in MBL-/- mice were significantly higher than those in WT mice. Conclusion MBL can inhibit the differentiation of CD4+T cells to Th17 cells, which is induced by IL-6 and TGF-β1.