Besides immune complexes which are products of antigen-antibody reactions, complexes of IgG of the non-immune nature could be found in the circulatory system. The non-immune complexes are the result of reactions of ligands of various origins with sites localized on the constant domains of IgG molecules. Shed cell-surface proteins or secreted proteins and products of cell destruction as well as various circulatory soluble proteins and peptides interact with IgG. By the association with IgG, proteins including potentially harmful ones can be eliminated from the circulation. Proteins and peptides absent in the liquid phase of the plasma or present only at very low concentrations (low abundant proteins) could be found as a part of the IgG complexes. Quantative studies of proteins bound by IgG in the circulation could give valuable clinical information on various types of pathology. In the review the methodological aspects, in particular methods of isolation and identification of proteins associated with IgG are discussed. Properties of some recently found circulated non-immune IgG complexes are summarized. Functional aspects of the complex formation are also discussed.