In adult male mice homozygous for the juvenile spermatogonial depletion (Utp14b^jsd) mutation in the Utp14b gene, type A spermatogonia proliferate, but in the presence of testosterone and at scrotal temperatures these spermatogonia undergo apoptosis just before differentiation. In an attempt to delineate this apoptotic pathway in jsd mice and to specifically address the roles of p53- and FasL/Fas receptor-mediated apoptosis, we produced jsd mice deficient in p53, Fas, or FasL. Already at the age of 5 weeks, less degeneration of spermatogenesis was observed in p53-null-jsd mice than in jsd single mutants, and in 8- or 12-wk-old mice the percentage of seminiferous tubules showing differentiated germ cells (tubule differentiation index, TDI) was 26–29% in the p53-null-jsd mice compared to 2–4% in jsd mutants with normal p53. The TDI in jsd mice heterozygous for p53 showed an intermediate TDI of 8–13%. The increase in the differentiated tubules in double mutant and p53 heterozygous jsd mice was mostly attributable to intermediate and type B spermatogonia; few spermatocytes were present. TUNEL staining showed that most of these differentiated spermatogonia still underwent apoptosis, thereby blocking further continuation of spermatogenesis. In contrast, the percentage of tubules that were differentiated was not significantly altered either in adult Fas null-jsd mice or in adult FasL defective gld-jsd double mutant mice as compared to jsd single mutants. Further, caspase 9, but not caspase 8 was immunochemically localized in the adult jsd mice spermatogonia undergoing apoptosis. The results show that p53, but not FasL or Fas, is involved in the apoptosis of type A spermatogonia before/during differentiation in jsd mice that involves the intrinsic pathway of apoptosis. However apoptosis in the later stages must be a p53-independent process.