The following studies were undertaken to establish whether FSH binds specifically to testicular cells. Human pituitary FSH (LER 1460-2) tritiated in the sialic acid moiety was incubated with testis tubules from 20-day-old rats. At a concentration of 3H-FSH in the medium of 2.6 × 10–8M binding of 18 ng FSH (3000 cpm)/ 100 mg testis tubules was observed and the apparent Kd was approximately 7 × 10–9M. 3HFSH binding was detectable at 2 min and continued to increase for at least 60 min. Moreover, the binding was testis specific since incubation of liver, spleen, heart or lung failed to produce significant accumulation of 3H-FSH. Immature testis bound more FSH than mature testis and binding was restricted to tubular cells since isolated interstitial tissue was completely inactive in the binding reaction. Significant binding occurred at 4° and was maximal at 32–37°, whereas no binding was noted at 60°. Specificity of the “receptor” was tested by peptide hormone competition against 3H-FSH. Unlabeled preparations with FSH activity (Human LER 869-2; Rat NIH-B-1; Ovine NIH-S-8; HMG; and Human Urinary FSH or LER-907) competed effectively whereas LH (Ovine NIH-S-16 and Human LER-960), ACTH, TSH, HCG, PMS, GH, insulin, prolactin, and serum albumin did not compete. Finally, subcellular fractionation of the testis preparation revealed that much of the bound 3H-FSH was localized in cell membrane fractions. These results suggest the presence of specific testicular binding sites for FSH and indicate that FSH may initiate a biochemical sequence of events by binding to the cell membrane. (Endocrinology90: 39, 1972)