1. Tropomyosin was prepared from fresh human myocardium and antisera raised in rabbits. A sensitive radioimmunoassay was developed for the detection of human cardiac tropomyosin in human sera down to levels of 1 ng/ml. 2. Values for human cardiac tropomyosin in normal patients ranged from less than 1 to 3 ng/ml. In 18 patients with acute myocardial infarction all had elevated tropomyosin levels ranging from 41 to above 200 ng/ml with a mean peak level of 101 ng/ml. In this study there were no false positive or false negative results. 3. In the initial stages of infarction the time course of appearance and peak levels of cardiac tropomyosin, total creatine kinase and creatine kinase MB isoenzyme were similar. Although total creatine kinase and creatine kinase MB isoenzyme levels were normal after 72 h in patients with single, uncomplicated infarction, cardiac tropomyosin levels were still significantly elevated above normal after this time, being 30-60% of peak values. 4. Radioimmunoassay of human cardiac tropomyosin may prove useful in the diagnosis and in the management of patients with acute myocardial infarction, particularly in the long-term postinfarction period.