Analysis of the culture media of p40-transfected COS cells indicated the presence of 40 kDa monomers and 80 kDa disulfide-linked homodimers. Examination of partially purified p40 recombinant proteins demonstrated that only the homodimer but not the monomer binds to the IL-12 receptor. Partially purified 80 kDa homodimer inhibited [.sup.125 I]IL-12 binding to PHA-activated human lymphoblasts with an IC.sub.50 of 80 ng/ml, which is similar to the IC.sub.50 value (20 ng/ml) for the human IL-12 heterodimer. Although neither the 40 kDa monomer nor the 80 kDa dimer could stimulate human PHA-blast proliferation, the 80 kDa dimer inhibited IL-12-induced proliferation in a dose-dependent manner with an IC.sub.50 of 1 .mu.g/ml. The IL-12 p40 subunit contains the essential epitopes for receptor binding, but they are only active when p40 is covalently associated with a second protein such as p35 or p40. When p40 is associated with the p35 subunit, the heterodimer acts as an agonist mediating biologic activity. When p40 associates with itself, the homodimer behaves as an antagonist.