The relationship of T cell receptor assembly, stoichiometry, and expression to T cell development in the thymus has been elucidated using a variety of in vitro and in vivo models. We have documented the stoichiometry of the multisubunit TCR complex as consisting of 1- TCRalpha, 1-TCRbeta, 2-CD3epsilon, 1-CD3gamma, 1-CD3delta, and 2 zeta protein chains. In addition we have found that immature thymocytes also express on their surface clonotype-independent CD3 complexes consisting of CD3 components associated with a surface form of the molecular chaperon calnexin and that signals transduced by this complex can regulate early thymocyte development. TCR expression levels was found to be quantitatively regulated in immature thymocytes by destabilization of nascent TCRalpha proteins in the ER of immature thymocytes. The molecular basis for this instability is not fully known but appeared to be related to impaired calnexin association in the ER. Positive selection of immature thymocytes into mature T cells involved increased transcription and synthesis of nascent TCRalpha proteins. Positive selection signals were transduced in immature thymocytes upon TCR engagement under conditions in which TCR aggregation did not occur, as TCR aggregation inhibited positive selection. And, perhaps most interestingly, we found a marked asymmetry in the signals required for CD4 versus CD8 commitment, in that CD8-commitment was strictly dependent upon MHC class I instructional signals whereas CD4 commitment appeared to occur by default.