The thymus provides signals that control the proliferation and differentiation of T lymphocytes and select the repertoire of T-cell specificities. Antibodies to CD3 molecules inhibit full rearrangement of T-cell receptor <latex>$\beta $</latex> chain genes in organ cultures of early embryo mouse thymus. Whether this effect is mediated through <latex>$\gamma \delta $</latex>CD3 expressing cells, which are present in small numbers at this stage, or through low amounts of CD3 on <latex>$\alpha \beta $</latex> precursor cells is unclear. A requirement for special gene rearrangement signals within the thymus is supported also by the observations that growth factors such as IL-2 and IL-4, although stimulating proliferation of precursor cells removed from the thymus, do not induce full T-cell receptor gene rearrangements. Recent studies show that newly formed thymic lymphocytes expressing <latex>$\alpha \beta $</latex>CD3 receptors are targets for negative selection (deletion) as a means of removing autoreactive cells. Signalling to immature thymocytes via the <latex>$\alpha \beta $</latex>CD3 complex induces the activation of endogenous endonucleases that cleave DNA into oligonucleosomal fragments. We suggest that the activation of this mechanism is the means by which autoreactive cells are removed.