Binding of human [ 125 I]luteinizing hormone to homogenates of luteinized rat ovaries is dependent upon time, temperature, and pH and is saturable. Injection of human chorionic gonadotropin in vivo or addition of unlabeled human chorionic gonadotropin or human or ovine luteinizing hormone in vitro inhibits binding, whereas follicle stimulating hormone or prolactin is without effect. The dissociation constant for binding of luteinizing hormone receptor is 0.79 nM. The number of binding sites is 94 femtomol/mg of wet weight. NaCl, KCl, MgSO 4 , and CaCl 2 at concentrations below 10 mM have no effect on binding, while all salts significantly inhibit binding at 150 mM. Binding of [ 125 I]luteinizing hormone to its receptors is destroyed by proteolytic enzymes and by phospholipase C and D. The total binding activity is quantitatively recovered in the 2000 × g pellet of the homogenate.