Polypeptides corresponding to stable, circulating degradation products of troponin I (TnI) are described. The fragments comprise a sequence of the N-terminus of native cardiac TnI with 95-115 amino acid and additionally include fragments lacking about the 20-30 N-terminal amino acids. Utilities of these fragments and antibodies thereto include sensitive detection of myocardial infarction and purification of antibodies sensitive to the detection of troponin I degradation products.