Tissue injury including myocardial infarction leads to a variety of changes in plasma proteins commonly referred to as "the acute phase response". In this report the concentrations of serum amyloid A protein (SAA) were measured serially in 6 patients with myocardial infarction and 4 with angina. SAA was found to be increased in all patients with infarction, but in no patients with angina. Significantly increased SAA levels were detected 12 hours after the peak level of creatine kinase, and the concentrations of SAA seemed to correlate to the amount of damaged tissue. The SAA-response was both faster and more extensive than the response of C-reactive protein (CRP), but the correlation between SAA and CRP was very good.