Programmed cell death, or apoptosis, is important in homeostasis of the immune system : for example, non-functional or autoreactive lymphocytes are eliminated through apoptosis, One member of the tumour necrosis factor receptor (TNFR) family; Fas (also known as CD95 or Apo-1), can trigger cell death and is essential for lymphocyte homeostasis(1,2). FADD/Mort1 (refs 3-6) is a Pas-associated protein that is thought to mediate apoptosis by recruiting the protease caspase-8 (refs 7, 8), A dominant-negative mutant of FADD inhibits apoptosis initiated by Fas and other TNFR family members(6,9-14). Other proteins, notably Daax, also bind Fas and presumably mediate a FADD-independent apoptotic pathway(15). Here we investigate the role of FADD in vivo by generating FADD-deficient mice. As homozygous mice die in utero, we generated FADD(-/-) embryonic stem cells and FADD(-/-) chimaeras in a background devoid of the recombination activating gene RAG-1, which activates rearrangement of the immunoglobulin and T-cell receptor genes, We found that thymocyte subpopulations were apparently normal in newborn chimaeras. Fas-induced apoptosis was completely blocked, indicating that there are no redundant Fas apoptotic pathways, As these mice age, their thymocytes decrease to an undetectable level, although peripheral T cells are present in all older FADD(-/-) chimaeras, Unexpectedly, activation-induced proliferation is impaired in these FADD(-/-) T cells, despite production of the cytokine interleukin (IL)-2, These results and the similarities between FADD(-/-) mice and mice lacking the P-subunit of the IL-2 receptor suggest that there is an unexpected connection between cell proliferation and apoptosis.