Cardiac inflammatory responses appear to play a pivotal role in scar formation after acute myocardial infarction. Monocyte chemotactic and activating factor (MCAF) monocyte chemoattractant protein-1 (MCP-1) is a cytokine with chemotactic activity for mononuclear phagocytes, but also for NK cells, T cells, mast cells, and basophils. To investigate the possible involvement of MCAF/MCP-1 in the pathogenesis, its course was studied in patients with acute myocardial infarction. Twenty-three consecutive patients with acute myocardial infarction and 18 patients with angina pectoris were studied. Cytokines were measured by enzyme-linked immunosorbent assay. Plasma levels of interleukin IL-1alpha, IL-1beta, and IL-2 were below the detection limit of our method. IL-6 and interferon-gamma were detected in 17.4%, and tumor necrosis factor-alpha in 13.0% of patients with acute myocardial infarction, but the frequency was not statistically significantly different from that in angina pectoris. The plasma level of MCAF/MCP-1 in myocardial infarction tended to increase at 3 h after the onset of chest pain (133 +/- 19 pg/ml, P= 0.06) and was significantly elevated at 9 h (143 +/- 20 pg/ml) when compared with that in angina pectoris (87 +/- 6 pg/ml, P<0.05). The MCAF/MCP-1 level remained increased during the 24-hours observation period (P<0.01), and maximum level (168 +/- 13 pg/ml) was seen at 24 hour. The level of MCAF/ MCP-1 correlated significantly with the plasma level of another chemokine, IL-8, at 12 h after the onset of chest pain (r=0.51, P<0.05), suggesting that common stimuli mediate the release of both cytokines in myocardial infarction. The identification of MCAF/MCP-1 as an inflammatory mediator in acute myocardial infarction suggests that mononuclear phagocytes may play an important role in the early stage of the disease.