Immunoglobulin M (IgM) is the first and the largest antibody produced in response to an antigen. IgM is involved in complement activation, therefore leading to opsonization of antigens and cytolysis. Anti-Human IgM serum is used in various ELISA and immunohistochemistry assays. The presence of IgM antibodies in a patient’s serum indicates recent infection, or in case of a neonate’s serum, indicates intrauterine infection. Anti-Human IgM is used in the development and manufacturing of various qualitative and quantitative assays based on RID, IEP, ELISA, latex enhanced immunoturbidimetric assays, ouchterlony double diffusion, western blot, and precipitin assay.