The serum creatine kinase (CK), aspartate transaminase (AST), lactic dehydrogenase (LD) and alpha-hydroxybutyric dehydrogenase (HBD) were determined before and 3, 6, 18, and 36 hours after cardiac catheterization and angiocardiography in 56 consecutive patients with ischaemic heart disease. Five of these patients whose serum enzyme levels were higher than normal before the procedure were excluded from the study. Forty-one of the remaining 51 patients had left ventriculography and also selective coronary arteriography. In these 41 patients (groups 1 and 2--see below), the mean serum CK levels increased after the procedure to exceed the upper limit of normal at every study interval. The mean serum AST, LD, and HBD levels generally remained within the normal range at all study intervals, though serum AST increased abnormally in 9 of the 41 patients (22%) and serum LD and HBD each increased above the normal limit in 2 of 41 patients (4.9%). In 24 patients (group 1) whose coronary arteriograms showed insignificant coronary narrowing (less than 75%) in any of the three major coronary arteries, the increase in serum CK was significantly higher than in 17 patients (group 2) with greater than 75% narrowings in at least one of the three major coronary arteries. However, the degree of serum CK elevation observed during the postangiographic period was much lower than that in another group of 30 consecutive patients with acute myocardial infarction. In 10 patients (group 3) who had the same procedure as groups 1 and 2 except without the selective coronary arteriography, the serum enzyme levels showed no noticeable increase after the procedure. The difference in postangiographic serum CK elevation between patients with and without selective coronary arteriography and the difference between group 1 (without significant coronory narrowing) and group 2 (with significant narrowing) strongly suggest that the raised serum CK levels represent some form of myocardial damage caused by the coronary arteriography, which, however, is different at least in degree from that of acute myocardial infarction.