The object of the present invention is to provide a method for more accurately assaying the enzyme activities of mCK isozymes and a method for separately assaying the enzyme activities of CK isozymes by separately assaying the enzyme activities of ubiquitous mCK (umCK) and sarcomeric mCK (smCK). The above object is attained by an assay using an antibody that specifically recognizes umCK protein. In addition, other anti-mCK antibodies (e.g., anti-smCK antibody) and/or anti-human CK-M-inhibiting antibody can also be used. The above antibody is a polyclonal antibody or a monoclonal antibody. As a result of these antibodies, a monoclonal antibody (U1-1881) that is capable of specifically recognizing human umCK and is produced by a hybridoma having a deposition number of FERM BP-8342 is provided.