NEW MATERIAL:A monoclonal antibody having specificity to human IgM, and specifically linking ability to chain on the surface of B cell.

USE: Capable of dyeing specifically B cell clearly without dyeing background by making it with a fluorescent substance or an enzyme, dyeing tissue. Useful for classification and diagnosis of leukemia such as B cell chronic lymphocytic leukemia of cancerated B cell, differential diagnosis for glomerulonepheritis.

PREPARATION: A different animal is immunized against IgM purified from human serum, an antibody-producing cell having the immunization is collected, it is fused with a myeloma cell (preferably cell strain of 8-azaguanine resistance) of different animal. An antibody-producing hybridoma having human IgM resistance (e.g., 2C12 strain, or 3F1 strain) is selected from the prepared hydridoma by enzyme linked immuno sorbent assay, and it is cultivated to give the desired antibody.