Co-immunization with altered peptide ligands suppresses production of antibodies to mouse glucose-6-phosphate isomerase. Sera were taken on day 28 from mice co-immunized with 10 μg glucose-6-phosphate isomerase hGPI325--339 and 50 μg each altered peptide ligand (APL), and the titers of anti-mouse GPI IgG and IgG isotype were analyzed by ELISA. Each symbol represents a single mouse. Data presented as mean optimal density ± standard deviation. *P < 0.01, **P < 0.005, ***P < 0.001 (Mann--Whitney U test).