The central event in the cellular immune response to invading microorganisms is the specific recognition of foreign peptides bound to major histocompatibility complex (MHC) molecules by the alpha beta T cell receptor (TCR). The x-ray structure of the complete extracellular fragment of a glycosylated alpha beta TCR was determined at 2.5 angstroms, and its orientation bound to a class I MHC-peptide (pMHC) complex was elucidated from crystals of the TCR-pMHC complex. The TCR resembles an antibody in the variable V alpha and V beta domains but deviates in the constant C alpha domain and in the interdomain pairing of C alpha with C beta. Four of seven possible asparagine-linked glycosylation sites have ordered carbohydrate moieties, one of which lies in the C alpha-C beta interface. The TCR combining site is relatively flat except for a deep hydrophobic cavity between the hypervariable CDR3s (complementarity-determining regions) of the alpha and beta chains. The 2C TCR covers the class I MHC H-2K(b) binding groove so that the V alpha CDRs 1 and 2 are positioned over the amino-terminal region of the bound dEV8 peptide, the V beta chain CDRs 1 and 2 are over the carboxyl-terminal region of the peptide, and the V alpha and V beta CDR3s straddle the peptide between the helices around the central position of the peptide.