C-reactive protein and four other acute phase reactant proteins of non-cardiac, origin (orosomucoid, alpha 1- antitrypsin, heptoglobin, and alpha 2- macroglobulin) were studied serially by laser immunonephelometric assay in sera from 17 patients with myocardial infarction. A similar comparison was made in 57 patients undergoing surgery and 72 patients with acute infection. C-reactive protein was consistently the most sensitive acute phase reactant in all three conditions. After myocardial infarction, a raised serum C-reactive protein level was found on admission in four patients before a rise in creatine kinase MB isoenzyme (CK MB). The peak C-reactive protein level was reached on the third post-infarct day and it then declined over seven days with a half-life similar to myocardial tropomyosin. Serial monitoring of serum C-reactive protein, in parallel with cardiac proteins of short half-life (CK MB) and long half-life (tropomyosin), provides maximal information for diagnosis and for detecting post-infarct complications.