The purpose of this study was to investigate the significance of the sequential changes in proinflammatory cytokines observed in the plasma of patients early after myocardial infarct: a rise in interleukin (IL)-1 beta (308 +/- 126 vs. 141 +/- 78 pg/ml, P < 0.01) between 0 and 2 h followed by an IL-6 peak (49 +/- 24 vs. 14.5 +/- 13 pg/ml, P < 0.01) 4-9 h later. No significant changes in tumor necrosis factor-alpha (TNF-alpha) were observed at this early stage. The linkage between IL-1 beta and IL-6 secretions is supported by 1) the ability of patient's plasma drawn early after myocardial infarction to induce IL-6 mRNA and protein synthesis in cells that may be potential targets in vivo (fibroblasts and endothelial cells), 2) suppression of this activity by antibodies against IL-1 beta, and 3) a delay between IL-1 beta and IL-6 peaks in vivo (4-9 h), which is similar to the time required for maximal IL-6 production in IL-1 beta stimulated target cells in vitro (6 h). This sequential signaling might serve as the basis for an amplification mechanism of proinflammatory cytokines. In fact, a much greater synthesis of C-reactive protein was observed in hepatocytes when stimulated with conditioned medium of fibroblasts or endothelial cells that had previously been incubated with plasma of patients. The results of our work strongly suggest that, by inducing IL-6 in potential target cells, IL-1 beta could act as the primary, but indirect, signal that stimulates acute-phase protein synthesis after myocardial injury.