The T cell antigen receptor on most mature T cells consists of at least seven chains (the variable, clone-specific alpha and beta chains, and five constant chains, CD3-gamma, -delta, -epsilon, and -zeta 2). These chains assemble rapidly after synthesis in the endoplasmic reticulum. In this paper we describe an additional protein termed TRAP (T cell receptor associated protein) that is transiently associated with at least some of the newly synthesized murine receptor chains. TRAP remains bound to receptor subunits as they assemble for about 10-20 min after synthesis. Rapid dissociation of TRAP ensues. This can be blocked by manipulations that inhibit endoplasmic reticulum to Golgi transport or with agents that inhibit organelle acidification. In mutant T cells that fail to synthesize the beta chains, the partial complex fails to reach the Golgi system. Despite this, TRAP dissociates with kinetics similar to those observed in the parental T cells. Thus, our studies indicate that the dissociation of TRAP occurs in a pre-Golgi compartment by a process that requires an acidic intraorganellar pH.