Binding of gonadotropin releasing hormone (GnRH, pyro-Glu1-His2-Trp3-Ser4-Tyr5-Gly6-Leu7-Arg8-Pro9-Gly-NH210) to its plasma membrane receptor is the first step leading to pituitary luteinizing hormone (LH) release. In the present study, we have prepared the ethylene glycol bis(succinimidyl succinate) (EGS) dimer of a GnRH agonist, D-Lys6-GnRH; that is, (D-Lys6-GnRH)-EGS-(D-Lys6-GnRH). The bridge length of the EGS is about 15A. This dimer stimulates LH release from pituitary cultures with full efficacy but slightly less potency than D-Lys6-GnRH indicating that the dimerization merely restricts the action of each molecule of D-Lys6-GnRH with respect to the other. When a concentration of the dimer, which alone is too low to evoke substantial LH release, is incubated with pituitary cell cultures in the presence of antibody (AB) against D-Lys6-GnRH, considerable potency enhancement occurs. LH release in response to the AB-dimer conjugate requires extracellular Ca2+ and is blocked by Pimozide. The monovalent form (i.e. reduced pepsin fragment) of AB is ineffective in stimulating release. The addition of antibody to the dimer appears to confer the ability to cross-link receptors and indicates that receptor microaggregation as such is sufficient to activate the effector system in these cells and evoke release.