We identified IgG reactive with human cardiac troponin-I (cTnI) in plasma and serum samples (N = 1930) from normal blood donors, and in sample cohorts characterized on the basis of clinical biomarkers associated with cardiac, infectious, and autoimmune diseases. cTnI and brain natriuretic peptide were the biomarkers chosen to reflect myocyte damage or left ventricular dysfunction, respectively. The infectious disease cohorts were serologically positive for antibodies to hepatitis B (natural infection), hepatitis C virus, and Chagas (i.e., T.cruzi). The autoimmune cohorts were represented by samples from diagnosed systemic lupus erythematosus (biomarker: dsDNA) and rheumatoid arthritis (biomarker: rheumatoid factor) subjects. The prevalence of IgG autoantibodies reactive with cTnI was high in the normal donor cohort (95/750, 12.7%). The prevalence in the other sample cohorts was not significantly different from that in the normal blood donors, with the exception of a slight increase in the rheumatoid factor cohort (28/137, 20.4%). The presence of anti-cTnI IgG in highly reactive samples was confirmed by inhibition with the antigen and further by screening with a library of peptides derived from the human cTnI amino acid sequence. Our data suggest that these autoantibodies are polyspecific, encompassing epitopes across the entire cTnI sequence, including the cardiac-specific amino terminal region.