The T-cell antigen receptor is a multisubunit complex consisting of at least seven chains. Based upon structural and genetic considerations, we have divided these chains into three groups. The α and β subunits (Ti) are the clonotypic chains responsible for antigen recognition. Three chains that are invariant among all T-cells define the CD3 complex. These include the CD3γ, δ, and ε chains. The ξ chain is a distinct component that, like the CD3 chains, is invariant among all T-cells. In the majority of receptors, ξ is found as a disulfide-linked homodimer. We have recently shown that approximately 10% of ξ is disulfide-linked to a chain which we have called η. A preliminary model has been proposed, suggesting that there are two subclasses of receptors, depending upon the presence within the complex of either the ξ-ξ homodimer or the ξ-η heterodimer. Evidence has been presented that these two subclasses may perform distinct signaling functions. In this paper the η chain is characterized to determine whether it is structurally related to the ξ chain and, in particular, whether it might represent a post-translational modification of ξ. We can identify specific antigenic epitopes that are shared by both ξ and η. However, not all antibodies raised against ξ can directly recognize η. The apparent molecular mass of η is 22 kDa, whereas ξ has a molecular mass of 16 kDa. We are unable to demonstrate any post-translational covalent modifications of η to explain the difference in apparent molecular weight. These include phosphorylation, glycosylation, or sulfation. Amino acid incorporation studies demonstrate that the amino acid composition of η is distinct from that of ξ. All of the η in a T-cell is found in association with the rest of the components of the T-cell receptor. In addition, our anti-η antibodies allow us to directly recognize human η, which has an apparent molecular mass of approximately 23 kDa. Thus, η and ξ appear to be related but distinct proteins, and we would propose that η is the second member of the ξ group of components of the T-cell receptor.