Bioactive IL-12 is a potent regulator of cell-mediated immune responses and key mediator of Th1 cell development. Bioactive IL-12 is secreted by activated B lymphocytes, dendritic cells and macrophages as a 70 kD heterodimeric glycoprotein comprised of disulfide-bonded 35 kD and 40 kD subunits. The disulfide-linked p40 homodimer can bind to IL-12 receptors and antagonize the activities of bioactive IL-12. The 7B12 antibody reacts with human IL-12 p70 heterodimer, but shows no reactivity with IL-12 p35 or p40 monomer. The 7B12 has high affinity to human IL-12 p70 heterodimer.