Measurements of eosinophil cationic protein in serum (S-ECP) have been made in patients with acute myocardial infarction. In spite of low numbers of blood eosinophils in the acute phase of the disease S-ECP levels fluctuated widely with often markedly raised levels. Peak levels of S-ECP were significantly correlated (P less than 0.001) to peak numbers of blood eosinophils, although the former usually occurred within the first 2-3 d of illness and the latter usually after the sixth day. Intravenous injection of 0.5 g methylprednisolone in healthy individuals reduced blood eosinophil counts for at least 24 h, but S-ECP levels remained within normal limits. These results suggest that ECP is released from eosinophils in vivo. The raised levels of S-ECP in patients with acute myocardial infarction are probably due to the active participation of eosinophils in the inflammatory process.