Interleukin 12 (IL 12) p35/p40 is a heterodimeric cytokine which plays a critical role in inflammation, immunity and tissue proliferation, and also plays a relevant function in T helper (Th) cell polarization and Th1 T-cell differentiation. IL-12 family members, IL-12p70, IL-23, IL-27 and IL-35, play an important role in influencing helper T-cell differentiation. EBV-induced gene 3 can be associated with the p35 subunit of IL-12 to form the EBI3/p35 heterodimer, also called IL-35. It has been shown that IL-35 has biological activity and able to expand CD4+CD25+ Treg cells, suppress the proliferation of CD4+CD25- effector cells and inhibit Th17 cell polarization. IL-35 has been shown to be constitutively expressed by regulatory T (Treg) cells CD4(+)CD25(+)Foxp3(+) and suggested to contribute to their suppressive activity. IL-35 is a crucial mediator which provokes CD4+CD25+ T cell proliferation and IL-10 generation, another well-known anti-inflammatory cytokine, along with TGFbeta cytokine. These studies suggest that IL-35, together with other successfully discovered cytokine inhibitors, represents a new potential therapeutic cytokine for chronic inflammation, autoimmunity and other immunological disorders.