Natural Killer T cells (NKT cell), a type of T cell that plays a significant role in immune response, produce a large quantity of INF-γ and IL-4 in response to phospholipids that present in CD1d molecules. In recent years, NKT are reported to play a part in diabetes and tumor immunity. Therefore, a technology allowing quantitative measurement of CD1d-positive NKT cells would be a useful tool for immunology and clinical laboratory examinations. In recent years, a flow cytometric method of measuring NKT cells specifically and quantitatively has been developed in which CD1d/phospholipid compound tetramer labeled by Phycoerythrin (PE) is used as a probe. T-Select Mouse CD1d Tetramer-SA-PE is a reagent created by tetramerizing biotinylated Mouse CD1d molecule with streptavidin labeled by phycoerythrin (PE). -galactosylceramide (-GalCer), a phospholipid separated and synthesized from sponge, combines with CD1d to activate human and mouse NKT cells. Thus it is possible to quantify human and mouse NKT cell populations by combining T-Select Mouse CD1d Tetramer-SA-PE with -GalCel. Measurement can be performed using isolated lymphocytes/monocytes.