CD1d is a membrane protein non-covalently bonded to β2-microglobulin (β2m) and shows high homology between human and mice. CD1d can present α-galactosylceramide (α-GalCer), a glycolipid extracted and isolated from the marine sponge, and this complex can activate human and murine CD1drestricted NKT cells. CD1d Tetramer-PE is a reagent prepared by tetramerization of complexes of CD1d and β2m by PE- or APC- labeled streptavidin. Binding this reagent to α-GalCer enables highly sensitive detection of CD1d-restricted NKT cells and can be combined with antibodies to study NKT cell function by flow cytometry.