Description: The II/41 monoclonal antibody reacts with the mu 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.