The pre‐TCR complex regulates the transition from CD4^–CD8^– double‐negative (DN) to CD4⁺CD8⁺ double‐positive (DP) thymocytes during T cell development. In CD45^{– / –} mice there is an accumulation of DN cells, suggesting a possible role for CD45 in pre‐TCR signaling. We therefore crossed CD45^{– / –} with Rag‐1^{– / –} mice to investigate the signaling functions of the CD3 complex in DN thymocytes. Remarkably, treatment of Rag‐1^{– / –} / CD45^{– / –} mice with a CD3 mAb caused maturation to the DP stage at only 3 % of the level measured in Rag‐1^{– / –} mice. Furthermore, ligation of the CD3 complex on Rag‐1^{– / –} / CD45^{– / –} thymocytes in vitro induced less tyrosine phosphorylation in specific proteins when compared to Rag‐1^{– / –} thymocytes. CD45^{– / –} mice were also crossed with pLGFA mice expressing a constitutively active form of the lck tyrosine kinase which restored the DN to DP transition to near normal levels. Our results are consistent with a model in which CD45‐activated p56^lck is critical for pre‐TCR signal transduction.