Rheumatoid arthritis (RA) patients can be divided into responder and nonresponder patients, based on peripheral blood derived natural killer T (NKT) cell reactivity to α-galactosylceramide (α-GalCer). Peripheral blood (PB) mononuclear cells (1.5 × 106 cells/well) from nine healthy control individuals and 13 RA patients, as well as synovial fluid (SF) mononuclear cells from 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 on day 14. Patients were considered nonresponders when the frequency of Vα24+Vβ11+CD3+ NKT cells derived from PB mononuclear cells was lower than 2% after 14 days of culture. Error bars indicate standard error of the mean. *P < 0.01.