From an analysis of the immunoglobulins of known structure we derive a list of 40 sites crucial for the conserved structure of the variable domains. We show that, with marginal exceptions, the sequences of the T‐cell alpha beta receptors contain, at sites homologous to these 40, the same or very similar residues. Thus the V alpha‐V beta dimer has a framework structure very close to that of the immunoglobulins. Further comparisons show that parts of the surface of the V alpha‐V beta framework are hypervariable. They also show that the loops that form the antigen‐binding site are similar in size to those commonly found in the immunoglobulins but have different conformations. Only limited sequence variations occur in the first loop of the antigen‐binding site in both V alpha and V beta. This, and their geometrical arrangement, suggest that they mainly interact with the MHC proteins.