Description_x000D_ Biochem/physiol Actions_x000D_ Interleukin-12 (IL-12) was identified as a factor secreted by human Epstein-Barr (EBV)-transformed B cell lines. IL-12, or Natural Killer Cell Stimulatory Factor (NKSF), is a 75 kDa disulfide-linked heterodimer of a 35 kDa subunit and 40 kDa subunit. IL-12 is produced predominantly by monocytes and NK cells and induces T cells and NK cells to produce IFN-γ. Human IL-12 is not active on mouse cells, but murine IL-12 is active on both murine and human lymphocytes. The biological activity of recombinant human IL-12 was measured by its ability to stimulate the proliferation of PHA-activated human T lymphoblasts. IL-12 has anti-angiogenic properties._x000D_ Physical form_x000D_ Lyophilized from a 0.2 μm filtered solution in phosphate buffered saline containing 0.25 mg bovine serum albumin.