The monoclonal antibody 11-26c reacts with mouse IgD of all haplotypes (e.g., IgDa, IgDb, IgDc), and does not react with other immunoglobulin isotypes.1-4 11-26c does not induce proliferation of mature B cells in vitro,5 nor does in vivo injection of the monoclonal antibody have any effect on activation of mature B lymphocytes.5