Complicated interdigitation-like junctions between the immature Sertoli cells were frequently observed in prepubertal testis (3--8-year-old). Tight or gap junctions could not be found. Subsurface cisternae appear in the testes of 7 and 8-year-old boys. In pubertal testis (11 to 13 years old), junctional specializations between Sertoli cells are composed of membrane fusions, bundles of microfilaments and associated cisternae. These tight junctions block the deep penetration of lanthanum into the seminiferous tubules. The lanthanum-filled Sertoli junctions show characteristic features of membrane fusions. In the seminiferous tubules spermatocytes were not present at 11 years of age but complete spermatogenesis was noted at 12-13 years of age. It is suggested that the human blood-testis barrier is estabilished shortly before or after the spermatogonia proliferate to give rise to primary spermatocytes.