Immature thymocytes express a pre-T cell receptor (pre-TCR) composed of the TCR beta chain paired with pre-T alpha. Signals from this receptor are essential for passage of thymocytes through a key developmental checkpoint in the thymus. These signals were efficiently delivered in vivo by a truncated form of the murine pre-TCR that lacked all of its extracellular immunoglobulin domains. De novo expression of the truncated pre-TCR or an intact alpha beta TCR was sufficient to activate characteristic TCR signaling pathways in a T cell line. These findings support the view that recognition of an extracellular ligand is not required for pre-TCR function.