Immunological reactions have been recognized in cardiac disease and immune complexes (ICs) have been suggested to be of pathogenetic significance in the occurrence of post-myocardial infarction syndromes. ICs of IgA, IgG, IgM class have been detected in myocardial infarction utilizing a C1qSP and an anti C3 assay. A poor concordance was found between the two methods. Early pericarditis was found to be associated with the presence of ICs, suggesting that ICs may play a role in developing pericarditis or that they may be considered a marker of pericarditis by an immunological mechanism.