One-dimensional sodium dodecyl sulphate polyacrylamide gel electrophoresis of myocardial proteins followed by Western blotting is a sensitive method for the detection of antiheart antibodies after cardiac transplantation. In a previous study we found that the majority of patients made antiheart antibodies after cardiac transplantation. It is possible that these antibodies were formed in response to cardiac damage caused during the surgical procedure rather than being specific to the transplantation situation. In this study we have evaluated the role of open cardiac surgery in the formation of antiheart antibodies for the first 9 months of the postoperative period using the Western blotting technique and correlated that with the development of post-pericardiotomy syndrome. Only 25% (9/36) of patients showed any increase in the pre-existing level of antiheart antibodies or developed antiheart antibodies with new reactivities. None of the patients in the study developed manifestations specific for post-pericardiotomy syndrome during the period of follow-up. The results support the contention that the high incidence of antiheart antibodies formed after cardiac transplantation is due to a humoral immune response to the presence of alloantigens on the grafted heart rather than as a result of the surgical procedure itself.