Description: The II/41 monoclonal antibody reacts with the μ heavy chain of mouse IgM. It does not react with other classes of mouse immunoglobulin including IgD, IgG or IgA. IgM is expressed intracellularly, during early stages of B lymphopoiesis, and then on the surface of more mature B cells in the bone marrow and peripheral B cells. Fluorochrome conjugated II/41 can be used as a detection secondary for mouse IgM.