Interleukin-27 (IL-27) is a cytokine with multiple roles in regulating the immune response, but its effect on human CD56(bright) and CD56(dim) NK cell subsets is unknown. NK cell subsets interact with other components of the immune system, leading to cytotoxicity or immunoregulation depending on stimulating factors. We found that IL-27 treatment results in increased IL-10 and IFN-γ expression, increased viability and decreased proliferation in both CD56(bright) and CD56(dim) NK cell subsets. More importantly, IL-27 treatment imparts regulatory activity to CD56(bright) NK cells, which mediates its suppressive function on T cells in a contact-dependent manner. There is growing evidence that CD56(bright) NK cell-mediated immunoregulation plays an important role in the control of autoimmunity. Thus, understanding the role of IL-27 in NK cell function has important implications for treatment of autoimmune disorders.