Both a two-site immunoradiometric assay and a two-site enzyme-linked immunosorbent assay for creatine kinase MM are described. Linearity, reproducibility and cross-reactivity of the assays are satisfactory. Creatine kinase MM incubated in a pH-controlled serum matrix loses its activity, but has its antigenic determinants affected as well. Of all the techniques used, only the immunoradiometric assay is capable of measuring part of the inactivated enzyme. Measurements with this assay on the sera of patients after a myocardial infarction show identical results for enzyme activity and creatine kinase protein quantity. The in vitro disappearance rate of creatine kinase activity is slow compared with the in vivo half-life of the enzyme. These two observations lead to the conclusion that creatine kinase is removed from the circulation long before it is inactivated in the blood stream.