The precursor T-cell receptors (TCRs) and B-cell receptors (BCRs) direct lymphocyte development to the mature T-cell and B-cell stage, respectively. Recent genetic and biochemical experiments reveal the striking parallel in structure and function of these receptors. They consist of TCR beta and BCR mu chains paired with surrogate TCR alpha and BCR light chains. Both receptors employ a two-component signal transduction unit: CD3 gamma epsilon for the pre-TCR, and CD79ab for the pre-BCR. Plasma membrane levels of pre-TCR/BCR complexes are kept extremely low, most probably by a mechanism involving specific retention in the endoplasmic reticulum. This mechanism may control the signalling activity of pre-TCR/BCR and therewith the lymphocyte differentiation process.