The T lymphocyte receptor for antigen and histocompatibility molecules is a molecular complex comprised of five polypeptide chains. Both the 49KD alpha and 43KD beta chains are immunoglobulin-like and thus contain variable domains responsible for ligand binding. In contrast, the 20-25KD T3 gamma, delta and epsilon chains are monomorphic structures presumably involved in transmembrane signalling. The alpha and beta subunits are disulfide bonded to each other and held in noncovalent association with the T3 chains. T3-Ti receptor crosslinking leads to conformational modification of a second T lineage specific molecule, termed the 50KD T11 structure and in turn, protein kinase C activation, elevation in intracytoplasmic free calcium and Na+/H+ antiport stimulation.