This study was undertaken with the aim of investigating humoral and cell-mediated immune response in acute myocardial infarction (AMI) as possible mechanisms involved in the infarction enlargement. Twenty three patients with first AMI and 15 healthy volunteers were examined. Of the AMI patients, 14 had extensive infarction (group A), while 9 patients had small infarction (group B). Immunologic analyses were performed at admission, and repeated after 3, 7, 14 and 21 days of the acute event. Following parameters were tested: number of CD3+, CD4+, CD8+ and CD20+ cells; serum IgG, IgA, IgM, C3, C4, immune complex and anticardiac antibody levels; polymorphonuclear cell (PMN) function (chemotaxis, phagocytosis, metabolic activity); leukocyte migration in vitro in the presence of water-soluble homologous heart extract. It was demonstrated that the number of B cells, serum IgG, C3, immune complex and anticardiac antibody levels were elevated from 7th-14th days after AMI. Concerning these parameters, however, no significant differences were obtained between group A and group B of AMI patients. Chemotaxis and metabolic activity of peripheral blood PMN, but not phagocytosis, were enhanced during AMI, again changes of PMN did not correlate with the extension of infarction. In contrast, leukocyte migration inhibition in vitro revealed that only patients with extensive AMI have developed positive reaction during the first 14 days after the onset of the disease, while leukocyte inhibition reaction appeared in patients with nonextensive AMI not earlier than the 21st day after the infarction. These findings demonstrate generation of immune reactivity during AMI and indicate that humoral immune response seems more likely to be an epiphenomenon related to tissue necrosis, while cell-mediated immune reactions could influence the extensiveness of cardiac damage.