μMT → F1 mice have serum antibodies similar to control animals. Serum levels of IgM (A), IgA (B), IgG1 (C), IgG2a (D), and IgG2b (E) antibodies were determined by quantitative isotype-specific ELISA (Southern Biotechnologies). Each bar represents one mouse, black bars (▪) are μMT → F1 and white bars (□) are control chimeras. Results are representative of 2 independent experiments.