Immunofluorescence studies (IF) were carried out in 60 patients with a recent myocardial infarction and with three control groups, comprising respectively 30 patients with acute or chronic coronary insufficiency, 16 with various heart diseases, and 60 healthy subjects. Circulating anti-heart antibodies were detected in 19 of the 60 patients with a recent myocardial infarction; in two cases with Dressler's post-infarction syndrome their tit re was as high as 1:160. In one case of myocardial infarction with a high antibody titre there were no signs of the post-infarction syndrome. This patient was on corticosteroid treatment for other reasons. In the control groups immunofluorescence was positive at the dilution of 1:10 in only one of the 106 subjects.