We report false-positive cardiac troponin (cTn) I results in the microparticle enzyme immunoassay (MEIA) using the AxSYM analyzer. We studied serum samples from 12 patients with positive rheumatoid factor but with no indication of myocardial infarction (MI); 2 also had positive antinuclear antibody (ANA) titers. Serum samples from 7 patients with positive ANA titers and negative rheumatoid factors also were studied. Total creatine kinase (CK) was run using a Hitachi 747 analyzer, cTnT using an Elecsys 2010 analyzer, and cTnI and CK-MB using an AxSYM analyzer. We observed no measurable cTnI and cTnT concentrations in 12 control samples or in specimens with positive ANA titers and negative rheumatoid factors. In contrast, samples from 7 of 12 patients containing rheumatoid factors had measurable cTnI concentrations. Four specimens showed cTnI concentrations more than 2.0 micrograms/L, the suggested diagnostic cutoff for MI. None of the specimens showed detectable cTnT. The concentrations of total CK and CK-MB were within normal ranges in all specimens. False-positive results were observed only with the MEIA for cTnI. This interference can be eliminated by using a polyclonal antisera against rheumatoid factor. The chemiluminescent assay for cTnI showed no detectable cTnI concentration in any specimen.