The binding of 12sI-labeled human luteinizing hormone (hLH) to the human luteal tissue of the menstrual cycle was examined. Displacement studies demonstrated that 125 I-hLH was specifically bound to the human luteal tissue. Specific binding of 125 I-hLH was demonstrated in all the corpora lutea examined except for two aged corpora lutea at the early proliferative phase of the cycle. The number of binding sites for hLH increased from the early to mid-luteal phase and decreased towards the late

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... teal phase. However, the apparent dissociation constant in each corpus luteum did not vary throughout the menstrual cycle. Furthermore, the effects of diethylstilbestrol diphosphate (DES) and prostaglandin F2a (PGF2a) priming on the binding of125I-hLH to the luteal tissue were investigated and the changes in hLH receptors were estimated by Scatchard plot. The binding sites for hLH decreased by DES priming and increased by PGF2a priming. In contrast, the apparent dis-