DURING lymphocyte development, cellular proliferation and positive and negative selection events ensure the production of T and B lymphocytes bearing highly diverse, but self-tolerant, repertoires of antigen receptors(1,2). These processes are initiated when engagement of growth-factor receptors, or the T-3,T-4 and B-5 lymphocyte antigen receptors, induces tyrosine phosphorylation of specific SH2- and SH3-domain-containing cytoplasmic proteins, including Vav(3,6,7). Here we show that vav(-/-) embryonic stern cells generate only limited numbers of immature and mature T and B lymphocytes in the RAG-2 blastocyst complementation assays. Furthermore, Vav-deficient T lymphocytes showed severely impaired antigen receptor signalling. Finally, we demonstrate that Vav-dependent signalling pathways regulate maturation, but not CD4/ CD8 lineage commitment, during T-cell-receptor-mediated positive selection of immature CD4(+)CD8(+) precursors into mature CD4(+)CD8(-) or CD4(-)CD8(+) T cells.