In 27 patients presenting with angina pectoris at rest and normal serum creatine kinase (CK) activity, cardiac myosin light chains (CM-LC), myoglobin (MG), and CK-B isoenzyme were determined in 7 serial blood samples by radioimmunoassays. Measurable amounts of CM-LC were found in at least one serum sample in 13 patients. MG was found to be elevated in 9, and CK-B in 8 of these patients. In the 189 serum samples determined, CM-LC were found more frequently elevated (21.7%) than MG (13.2%, P less than 0.05) or CK-B (12.2%, P less than 0.05). Coronary angiograms were obtained in 21 of the 27 patients. Elevated marker protein concentrations were found only in patients with coronary artery stenosis greater than or equal to 70% of at least one coronary artery. The incidence of elevated serum concentrations of any of the 3 marker proteins determined was higher in patients with 3 vessel disease than in those with 1 or 2 vessel disease (33.9% vs 15.6%, P less than 0.05), and it was higher in patients with a history of previous myocardial infarction than in those without (34.5% vs 11.4%, P less than 0.001). The findings suggest that in a subgroup of patients with angina pectoris at rest but without evidence of acute myocardial infarction, ischaemic damage of small myocardial areas can be detected by serological assays of high sensitivity. Among the marker proteins studied, CM-LC were found the most sensitive.