Differences in serum immunoglobulin IgG2a and IgG2b isotypes are abolished in post-pubertal B6 lpr (Fas-deficient) and B6 gld (FasL-deficient) mice. Serum samples from 16- to 20-week-old male and female (5–6 mice/sex) B6, B6 lpr and B6 gld mice were assayed by ELISA. The samples were diluted at 1:5,000 (IgG2a); 1:10,000 (IgG3); 1:50,000 (IgG1 and IGg2b). Bar graphs represent mean absorbance at 405 nm ± SE. Significant differences for IgG2a (p = 0.0009) and IgG2b (p = 0.00005) between post-pubertal male (blue bar) and female (checker red bar) mice are shown with asterisks.