A study of 116 patients admitted on the suspicion of myocardial infarction is presented. Twenty three of the patients were found to have infarction. For the diagnosis of infarct the most reliable biochemical analyses at present appear to be serum creatine kinase (S-CK, E.C.188.8.131.52.) with its isoenzyme S-CKMB and serum lactate dehydrogenase (S-LD, E.C.184.108.40.206.) with its isoenzyme S-LD-1. If an organization with blood samples at fixed intervals after the appearance of symptoms could be arranged, this would be most recommendable. In many hospitals this is, however, difficult and the blood samples have to be drawn at specified hours. In the four hospitals in the county of Västernorrland, Sweden, this latter procedure has been introduced. Blood is drawn at the arrival of the patient, at 8 am and 8 pm the first day, and at 8 am the next two days of the stay in hospital. In the first two samples S-CK and S-LD are determined, in the third S-CK, in the fourth S-CK and S-LD and in the fifth S-CK. If the values are elevated, S-CKMB and S-LD-1 are determined. S-Myoglobin has been found valuable in the early exclusion of the diagnosis of infarction.