According to Jerne's idiotypic network hypothesis, the adaptive immune system is regulated by interactions between the variable regions of antibodies, B cells, and T cells.The sym. immune network theory is based on Jerne's hypothesis, and provides a basis for understanding many of the phenomena of adaptive immunity.The theory includes the postulate that the repertoire of serum IgG mols. is regulated by T cells, with the result that IgG mols. express V region determinants that mimic V region determinants present on suppressor T cells.In this paper we describe rapid binding between purified murine serum IgG of a given strain and IgG of the same strain and also between IgG from MHC-matched mice, but not between serum IgG preparations of mice with different MHC genes.We interpret this surprising finding in terms of a model in which IgG mols. are selected to have both anti-anti-(self MHC class II) and anti-anti-anti-( self MHC class II) specificity.