The pathogenesis of post-cardiac injury syndrome was studied prospectively in 62 patients who underwent coronary bypass grafting. Preoperative and serial postoperative titres of actin and myosin antibodies were measured by an enzyme linked immunosorbent assay. Perioperative cumulative release of serum aspartate and alanine aminotransferases, lactate dehydrogenase, and creatine kinase was calculated by approximation formulas that are used to estimate infarct size. Complete post-cardiac injury syndrome developed in eight (13%) patients and an incomplete syndrome developed in 16 (26%). There was a significant correlation between frequency and intensity of the syndrome and the ratio of postoperative to preoperative titres of actin and myosin antibodies. Furthermore, there was a significant correlation between the cumulative release of lactate dehydrogenase, serum aspartate aminotransferase, and creatine kinase and the number of coronary vessels that were grafted, but no correlation was found between the incidence of post-cardiac injury syndrome and the number of coronary bypasses grafted or between the cumulative enzyme release and the postoperative immunological response against the major contractile proteins, actin and myosin. The amount of enzymes released during coronary bypass surgery seems to be a good indicator of the extent of myocardial damage during operation but it does not determine either the incidence of post-cardiac injury syndrome or the postoperative immunological response against the main contractile proteins actin and myosin.