It has been well known that the determination of CK-MB is a useful parameter for the diagnosis of acute myocardial infarction (AMI). Currently, several methods such as Electrophoresis, Ion-exchange chromatography, Immunoinhibition method and Protein mass determination are available. From a point of the diagnosis of AMI, the method should be satisfied with the requirements of high speed, ease to perform, high analytical specificity and high analytical sensitivity. However, there are no appropriate methods which can satisfy all of the requirement. In this paper, we have introduced the present situation concerning the determination of CK-MB in our hospital and discussed the results of study on the IMx method based on the determination of protein mass of CK-MB. We obtained good results in the fundamental study and found some advantages of protein mass determination. The first advantage was its high analytical specificity by use of monoclonal antibody. The second advantage was its high analytical sensitivity because of its low detection limit. The third advantage was the stability of sample because the antigenicity of CK-MB to the antibody of reagent is stable comparing with that of enzyme activity. From those advantages, the mass determination of CK-MB will be a useful method for the diagnosis of AMI in the near future.