Method for assaying creatine kinase isozyme activity and assay reagent

In a method for measuring the activity of creatine kinase MB isozyme in a fluid sample of a patient wherein an inhibition method is used to measure the enzymatic activity of creatine kinase (CK) isozymes, said inhibition method comprising contacting with an inhibitory antibody to human CK-M subunit to eliminate the CK-M subunit therein determining the activity of the remaining CK-MB subunit, the improvement comprising contacting the sample with an inhibitory antibody that binds to CK-M subunit (CK-M) to eliminate the enzymatic activity of the CK-M subunit and an inhibitory antibody that binds mitochondrial CK (m-CK) is used to eliminate the enzymatic activity of m-CK, and thereafter determine the enzymatic activity of remaining creatine kinase isozymes as the CK-MB isozyme in the sample.