We describe a novel signal-transducing protein complex, which consists of the T cell receptor (TCR) beta chain that is disulfide linked to a 33 kd glycoprotein and noncovalently associated with proteins of the CD3 complex on the surface of the pre-T cell line SCB.29. This 33 kd glycoprotein, provisionally designated gp33, represents neither of the known TCR chains and has escaped previous detection because it labels poorly by surface iodination. This glycoprotein is absent from the surface of mature T cell lines. A TCR beta complex with identical molecular masses before and after reduction can be immunoprecipitated from surface-iodinated large thymocytes of TCR alpha-deficient mice. The novel gp33-TCR beta complex may be entirely or partly responsible for control of early T cell development exerted by the TCR beta protein.