The goal of this study was to determine whether class I proteins play an important role in the regulation of Ig and to elucidate the mechanism(s) involved. We analyzed the phenotype imposed by a null allele of beta 2-microglobulin (beta 2m). Serum Ig levels of several mouse strains showed a beta 2m dependence that was most evident in mice genetically predisposed to develop chronic systemic lupus erythematosus, was preferential to IgG isotypes, and was greatly exaggerated in aging mice that normally develop hypergammaglobulinemia. Beta 2m-deficient mice, regardless of genetic background, also displayed a substantial reduction of specific Ab in response to a prototypic T cell-dependent Ag and a prototypic T cell-independent 2 Ag. This reduction could be accounted for by a selective diminution of Abs of the IgG class. Therefore, class I proteins play a considerable role in the regulation of Ig. The beta 2m dependence could not be explained by class I-dependent immunoregulatory cells (CD8+ cells, NK1.1+ T cells, or conventional NK+ cells) or by the transfer of maternal IgG into the prenatal/neonatal mouse made possible by the beta 2m-dependent Fc receptor (FcRn). However, a beta 2m-dependent increase in the half-lives of IgG, presumably conferred by lifelong FcRn expression, was observed in all mice regardless of genetic background and age. We conclude that FcRn-mediated protection of IgG from catabolism is a generic mechanism that best explains the lifelong beta 2m dependence of Ig in both normal and pathologic situations.