A method for determination of the creatine kinase isoenzyme MB (CK-MB) is reported: separation of the isoenzymes was done by electrophoresis and the activity of the isoenzyme bands quantitated by scanning fluorometry. Total CK activity was used for calculation of CK-MB level. The precision of the method was satisfactory: coefficient of variation 5-10%. Its accuracy good: CK-MB was consistently found in high concentrations in tissue extracts of myocardium, but was virtually absent in skeletal muscle and could not be demonstrated in serum from patients with skeletal muscle damage. The sensitivity of the method fitted its clinical use: CK-MB was undetectable (less than 5 U/l) in normal sera, below 30 U/l in seventy-six out of seventy-seven patients in whom the diagnosis of acute myocardial infarction (AMI) was disproved, and above 30 U/l in all seventy-two patients with AMI according to WHO criteria. The CK-MB concentration in serum rises to a maximum about 20 h after onset of clinical symptoms of AMI and reaches baseline levels 20-30 h later. The electrophoretic CK-MB method is easy, fast and reliable and is considered as an important diagnostic test for AMI.