Serum total immunoglobulin G (IgG) and rheumatoid factor (RF) are increased in CD1d-deficient (CD1d°) BWF1 mice. CD1d° BWF1 mice and control littermates (CD1d+/+ or CD1d+/-, designated as CD1d+) were bled and serum samples tested for total IgG and RF levels. (a) Eight-month-old CD1d+ (n = 7) and CD1d° (n = 13) mice (*P = 0.04). (b) CD1d+ (n = 8) and CD1d° (n = 15) mice (*P <0.01 and **P = 0.03 to 0.06). The negative control mean ± SD value in six normal BALB/c mice was 28.2 ± 8.6 U/ml for RF. Results are expressed as the mean ± standard error of the values.