Defects in natural killer T (NK T) cell function and of interleukin-4 -production in SJL and NOD mice have been linked to susceptibility to autoimmune disease. As SJL and NOD mice both carry the T-cell receptor (TCR) -chain locus c (Tcrac) haplotype, found in few other strains, we have attempted to determine the influence of Tcra polymorphism on NK T-cell recognition of ligand, selection, and immune responses. The majority of NK T cells use an invariant TRAV11J15 (previously called AV14J18 or V14J281) - chain paired with either TRBV132, BV29, or BV1 to recognize ligands presented by mCD1 molecules, including the glycolipid -galactosylceramide (-GalCer). Sequencing of TRAV11 from the mouse strains B10.A (encoding the Tcrab haplotype), B10.A-Tcrac, and NOD (Tcrac) shows that Tcrac has a single TRAV11 gene (TRAV11*01) and that Tcrab has a single expressed gene (TRAV11*02), plus a closely related pseudogene. There is no apparent difference in -chain J-region usage or in the CDR3 sequence at the TRAV11-J15 junction between the haplotypes in TRAV11-bearing NK T cells. Using Biacore and tetramer-binding and decay assays, we have determined that the interaction between Tcrac TRAV11*01 NK T TCR and the mCD1/-GalCer complex is slightly weaker than that of Tcrab (i.e., TRAV11*02) NK T TCR. These differences are minor compared with differences between agonist and antagonist ligands in other TCR systems, suggesting that it is unlikely that TCR polymorphism explains the defect in NK T cells in the autoimmune mouse strains.