PURPOSE: To provide a monoclonal antibody, having specificity to the isozyme of the heavy chain of myocardial myosine, useful as an important reagent for the biochemical and pathological research of cardiac muscle, and enabling the topographic diagnosis of myocardial infarction.

CONSTITUTION: Human atrial myosine (α-type) or human ventricular myosine (β-type) immunized to a different animal, and the antibody-producing cell, e.g. immunized spleen cell, lymphonodus cell, etc. are collected. The cell strain resistant to drug, unsurvivable in a selected medium in unfused state, and survivable in fused state (preferably cell strain resistant to 8-azaguanine) is used as the myeloma cell, and fused with the antibody-producing cell at a mixing ratio of 1:4W10. The antibody-producing hybridoma is separated from the fused cell in a selected medium, and cultured e.g. in the RPMI 1,640 medium containing 10W 15% bovine fetus serum. The monoclonal antibody having specificity to only one of α-type or β-type is separated from the supernatant liquid of the cultured product.