THE protein Lck (p56^lck) has a relative molecular mass of 56,000 and belongs to the Src family of tyrosine kinases^1–3. It is expressed exclusively in lymphoid cells, predominantly in thymocytes and peripheral T cells^4,5. Lck associates specifically with the cytoplasmic domains of both CD4 and CD8 T-cell surface glycoproteins^6,7 and interacts with the β-chain of the interleukin-2 receptor⁸, which implicates Lck activity in signal transduction during thymocyte ontogeny^9–13 and activation of mature T cells^14–19. Here we generate an lck null mutation by homologous recombination in embryonic stem cells to evaluate the role of p56^lck in T-cell development and activation. Lck-deficient mice show a pronounced thymic atrophy, with a dramatic reduction in the double-positive (CD4⁺CD8⁺) thymocyte population. Mature, single-positive thymocytes are not detectable in these mice and there are only very few peripheral T cells. These results illustrate the crucial role of this T-cell-specific tyrosine kinase in the thymocyte development.