Serum immunoglobulin (Ig) levels in β2 microgloblin-deficient (β2m°) BWF1 mice: reduced polyclonal total IgG, but not IgM. β2m+/+ (n = 18 to 22 female and 12 male, closed circles), β2m+/- (n = 30 to 35 female and 28 to 35 male, triangles) and β2m° (n = 11 to 16 female and 10 to 16 male, open circles) mice were bled and sera tested for total IgG, IgG2a and IgM. Results are expressed as the mean ± standard error. (a, b) Serum levels of total IgG and IgG2a isotype were lower in β2m° mice than in control animals (*P <0.05, **P <0.01 to <0.001; Student's t-test). (c) Serum levels of total IgM were higher in β2m° than in β2m+/+ mice (*P <0.05, Student's t-test).