In order to establish the ultrastructural features of Leydig cells 50 biopsies or prepubertal human cryptorchid testes have been examined. Light and electron microscopic observations reveal the presence of two types of interstitial cells distinguishable on the basis of the peculiar ultrastructural characteristic of the nucleus and cytoplasm. In our opinion these two types of interstitial cells seem to represent different stages of cell differentiation. There are fibroblast-like cells and cells corresponding to Leydig cells. From our observations it may be inferred that the fibroblast-like cells are the precursor cells of the Leydig cells.