C57BL/6 (B6)-jsd/jsd male mice are sterile because of lack of spermatogenesis. To find the cause of the deficient spermatogenesis, we have examined whether the mutation phenotype is the result of a defect in germ cells or in supporting cells using germ cell transplantation. In the seminiferous tubules of B6-jsd/jsd mutant mice, donor germ cells derived from the wild type GFP transgenic mouse (B6-+/+GFP) were able to undergo complete spermatogenesis, indicating that the juvenile spermatogonial depletion (jsd/jsd) mouse possesses normal supporting cell functions. In contrast, undifferentiated spermatogonia derived from B6-jsd/jsd mice were unable to differentiate in the seminiferous tubules of W/W v mice, even if the mutant germ cells successfully settled in the tubules. These results demonstrate that the deficiency in spermatogenesis of B6-jsd/jsd mice can be ascribed to a defect in spermatogonia but not in their supporting cell environment. Furthermore, the defect in B6-jsd/jsd spermatogonia is not in their ability to proliferate, but in their differentiation and may result from their hypersensitivity to high concentrations of androgen in the testis.