Following microdissection of the adult rat testis, the intertubular fraction rich in interstitial cells was incubated in vitro with 125Ilabeled human luteinizing hormone (HLH). Radioautography performed on interstitial cells after this incubation revealed cytoplasmic localization of 125I-HLH. Further in vitro studies indicated that both testicular and renal homogenates bound 131I-HLH and prior incubation with unlabeled HLH decreased the testicular and renal binding. However, the binding of 131I-HLH to testicular homogenates was of greater magnitude than the binding of this material to renal homogenates. These data support the concept that the interstitial cells of the testis contain a system of cytoplasmic receptor sites capable of detecting small quantities of HLH. (Endocrinology80: 332, 1971)