The T cell antigen receptor (TCR) is a multisubunit surface molecule on T cells which recognizes foreign antigens. In addition to the clone-specific alpha beta or gamma delta heterodimer antigen recognition element, each TCR has five invariant chains--the CD3-gamma, -delta, and -epsilon chains and a zeta zeta or zeta eta disulfide dimer. Receptor assembly and surface expression requires the presence of all chains except eta. Targetting of partial complexes, however, is determined differently by specific chains with the zeta chain in murine T cells providing safe transport of assembled pentamers from the Golgi complex to the cell surface. TCR signalling involves activation of two kinase pathways--protein kinase C and a non-receptor protein tyrosine kinase. zeta eta-containing TCRs couple preferentially to the PKC pathway by mediating phosphoinositide hydrolysis. We have evidence that the activated protein tyrosine kinase may be fyn, a member of the src family. While specific signalling roles for all invariant chains are not yet defined, we have implicated the zeta chain as uniquely coupling TCR antigen engagement to distal IL-2 signalling, perhaps via activation of the tyrosine kinase pathway.