Adenosine deaminase (ADA) catalyzes the conversion of adenosine and deoxyadenosine to inosine and deoxyinosine, respectively. ADA-deficient individuals suffer from severe combined immunodeficiency and are unable to produce significant numbers of mature T or B lymphocytes. This occurs as a consequence of the accumulation of ADA substrates or their metabolites. dATP is a candidate toxic metabolite because its concentration in RBCs of ADA-deficient patients correlates with the severity of disease. Murine fetal thymic organ culture (FTOC) under ADA-deficient conditions can be used as a model system to investigate the biochemical mechanism responsible for the inhibition of thymopoiesis. In ADA-deficient FTOCs initiated at day 15 of gestation, thymocyte development was arrested at the CD4 −CD8 −CD44 loCD25 + to CD4 −CD8 −CD44 loCD25 − transition. Apoptosis appeared to be involved because the cultures could be rescued by the pan-caspase inhibitor zVADfmk, a Bcl-2 transgene, or deletion of apoptotic protease activating factor-1. As in ADA-deficient patients, dATP was also elevated in ADA-deficient FTOCs. dATP levels were normalized and thymocyte development was rescued in cultures treated with an inhibitor of adenosine kinase, the enzyme that phosphorylates deoxyadenosine to dAMP. zVADfmk also prevented the accumulation of dATP in ADA-deficient FTOCs, suggesting that deoxyadenosine was derived from thymocytes undergoing apoptosis as a consequence of failing the β selection checkpoint. In contrast, dATP levels remained elevated in ADA-deficient FTOCs with fetal thymuses from Bcl-2 transgenic mice. These data suggest that thymocyte apoptosis as a consequence of failing developmental checkpoints involves one or more caspases that are not regulated by Bcl-2.