Thirty-one patients with proven acute myocardial infarction (MI) were studied prospectively at the time of admission to hospital and at 3, 7 and 18 days using 4 immune complex (IC) assays. Each assay showed an increased incidence of IC activity in MI with 76% of patients being positive in at least 1 assay on one or more of the sampling days. A positive IC assay did not show a significant correlation with cardiac failure, pericarditis, post MI syndrome or previous infarction. The presence of IC was found to correlate with serum C-reactive protein (CRP), serum enzymes and ESR and suggested that complexed CRP or other acute phase proteins may account for some of the IC activity found with less specific assays. The measurement of IC levels in MI has not proved helpful in the diagnosis, management or prediction of outcome in this disorder.