Interleukin-12 (IL-12) is a potent regulator of cell-mediated immune responses. Biologically active IL-12 is secreted by activated B lymphocytes and macrophages as a 70 kD heterodimeric glycoprotein comprised of disulfide-bonded 35 kD (p35) and 40 kD (p40) subunits. The IL-12 p40 monomer shares amino acid sequence homology with the IL-6 receptor. It has been reported that activated PBMC produce a manyfold excess of IL-12 p40 monomer over the bioactive p70 heterodimer. The IL-12 p40 monomer has been reported to inhibit binding of IL-12 p70 to the IL-12 receptor, but with 20X less effectiveness than the IL-12 p70 homodimer. Recombinant human IL-12 p70 (Cat. No. 554613) is supplied as a frozen liquid comprised of 0.22 μm sterile-filtered aqueous buffered solution, containing bovine serum albumin and glycerol, with no preservatives. Purity is ≥ 95% as determined by non-reducing SDS-PAGE, and an absorbance assay based on the Beers-Lambert Law. The endotoxin level is ≤ 0.1 ng per μg of human IL-12 p70, as measured in a chromogenic LAL assay.