Gonadotropin-releasing hormone (pyroGlu1-His2-Trp3-Ser4-Tyr5-Gly6-Leu7-Arg8-Pro9-Gly10 amide, GnRH) stimulates pituitary luteinizing hormone (LH) release. An antagonist, D-pyroGlu1-D-Phe2-D-Trp3-D-Lys6-GnRH (GnRH-Ant), binds to the pituitary GnRH receptor and inhibits GnRH-stimulated (10−9 M) gonadotropin release from pituitary cultures (IC50 = 2 × 10×7 M). GnRH-Ant has no measurable agonist activity at concentrations up to 10−6 M. The presence of the D-Lys6 both affords protection against proteolysis and includes an amino group which may be used for derivatization without loss of receptor-binding activity. Formation of the GnRH-Ant dimer by cross-linking of the (lysyl) amino groups of two molecules with ethylene glycol bis(succinimidyl succinate) (EGS) results in a GnRH-Ant dimer joined by a 12–15 A chain. As the amino terminus on GnRH-Ant is blocked, leaving the D-Lys6 amino group the only reactive group1, the reaction of EGS with GnRH-Ant does not lead to larger polymers. Like the parent compound, this dimer is purely an antagonist. We now show, however, that when antibody (AB) to D-Lys6-GnRH (which cross-reacts with GnRH-Ant) is incubated with excess dimer, a product is formed which consists of a divalent antibody with a GnRH-Ant dimer attached to each arm: AB–((GnRH-Ant)–EGS–(GnRH-Ant))2. In contrast to the parent compounds, this conjugate is an agonist, stimulating LH release from pituitary cultures. Our results suggest that a pure antagonist becomes an agonist when it is capable of bringing two receptor molecules within a critical distance, d (15 A