Elevated adenylate kinase activities in serum and urine have been studied among patients suffering from myocardial infarction. 71% of the patients had clearly increased activities in serum already on admission to the hospital, i.e. about 12 hours after onset of symptoms and patients was 45%. A maximum peak value of the activity in serum was seen at 6 hours after admission to the hospital, i.e. about 12 hrs after onset of symptoms and the elevated enzyme activity value persisted for at least 12 hours after admission. The patients with the elevated adenylate kinase activity in serum were arbitrarily divided into three groups as regards activity. It was shown that the difference in activity of ASAT as well as LD of the high and low activity group was significant (p less than 0.01). No such difference was established between these groups for ALAT, usually not being considered as a reliable test enzyme for myocardial infarction.