CpG ODN enhances production of antigen-specific antibody of IgG2a isotype. Mice were immunized with a single dose using various combinations of Id-KLH, GM-CSF, Id/GM-CSF fusion protein, and CpG ODN. Serum was obtained 4 weeks after a single immunization. Anti-Id IgG1 and IgG2a was determined by ELISA. Three mice were included in each group. (▨) Anti-Id IgG2a; (▪) anti-Id IgG1.