[125I]-(GnRH) concentration dependence binding curves using isolated bovine anterior pituitary plasma membranes, intact and solubilized, and purified GnRH receptor protein, are compared. In all instances the concentration dependence binding curves had a stepwise character here interpreted as multisigmoid, with several steep increases and plateaus. These curves are compatible with the existence of several binding sites for GnRH. Purification of the GnRH receptor protein (GnRH.R) resulted in about 500 000-fold increase of binding activity and yielded a single protein species on polyacrylamide gel electrophoresis in SDS, of estimated molecular weight 60 000. Similarity of GnRH binding by the purified protein with that of intact and solubilized plasma membranes suggested that a single protein was responsible for the binding in each instance. Thus heterogeneity of GnRH binding is likely attributable to phase transitions of a single receptor protein into different allosteric forms. The data suggest that positive cooperativity is involved in the studied system.