We used Western-blot analysis to investigate the possible presence in the bloodstream of the contractile protein alpha-actin in 70 patients diagnosed with acute myocardial infarction on the basis of clinical, electrocardiographic and laboratory (creatine kinase and lactate dehydrogenase) criteria. Circulating protein was identified with a monoclonal antibody specific for cardiac alpha-actin. Of the 70 control samples of blood, the immunoblot results were negative for alpha-actin in 98% of the cases. Of the 30 patients with skeletal muscle damage caused by surgery, 26 were negative for circulating alpha-actin. Of the 70 patients with acute myocardial infarction, circulating alpha-actin was found in 67 (95%) as a 43 kDa band in immunoblots; the highest circulating concentrations (0.0580 micrograms/microliters) were found in those with anterior acute myocardial infarction. Circulating alpha-actin was detected in samples taken between 1 and 180 h after the onset of pain, and showed a biphasic pattern of appearance. Our findings for serum alpha-actin, together with the relationship between serum concentrations of this protein and sex (p = 0.001), tobacco use (p = 0.007) and postepisode complications (p = 0.002), should make it possible to gain a deeper understanding of acute myocardial infarction as a clinical entity.