Five hybridomas producing McAbs against human serum IgM-isotype were obtained by fusing the myeloma NS-1 cell and the RALB/C murine spleen cell which had been immunized by serum IgM of patient with B chronic lymphocytic leukemia (B-CLL). These McAbs only reacted with human IgM, not with other immunoglobulins in ELISA and immune double diffusion test. An approximate positive rate of peripheral blood lymphocyte (PBL) was got when these McAbs and the McAbs aginst B cell were tested by indirecl immunofluorescent assay. The positive rate was similar to that obtained by direct immunofluorescent test. Immunoblotting showed that the molecular weight of the antigen to these McAbs was 70–90 Kd, indicating that the antigen was IgM. The practical value of the McAbs against human IgM was also discussed.