Reactivity of peripheral blood (PB) and synovial fluid (SF) derived natural killer T (NKT) cells to α-galactosylceramide (α-GalCer). PB mononuclear cells (1.5 × 106 cells/well) of nine healthy control individuals and 13 rheumatoid arthritis (RA) patients as well as SF mononuclear cells of five RA patients were stimulated with α-GalCer and re-stimulated on day 7 with autologous, α-GalCer pulsed, irradiated PB mononuclear cells in the presence of 2 U/ml IL-2. NKT cell numbers were determined by flow cytometry at day 14. Error bars indicate standard error of the mean. *P < 0.01.