The autoimmune-type Fcgr2b with deletion polymorphism in AP-4-binding site in the promoter region is suggested to be one most plausible susceptibility gene for systemic lupus erythematosus (SLE). We previously found that there is a strong epistatic interaction between the autoimmune-type Fcgr2b polymorphism and Y chromosome-linked autoimmune acceleration (Yaa) mutation, thus severe SLE observed in BXSB males neither develops in BXSB females nor in the congenic BXSB.IIB(B6) males carrying wild C57BL/6-type Fcgr2b. Present studies examined whether the wild-type Fcgr2b could suppress SLE in mice carrying Yaa-unrelated SLE susceptibility genes. Comparison of disease features between SLE-prone (NZW x BXSB) F1 females and the congenic (NZW x BXSB.IIB(B6)) F1 females carrying wild-type Fcgr2b showed that, as compared with findings in the former, SLE features including activation/proliferation of not only B cells but also T cells and monocytes/macrophages were all inhibited in the latter. It was concluded that the autoimmune-type Fcgr2b promotes and the wild-type inhibits SLE through mechanisms that promote and suppress activation/proliferation of a wide variety of immune cells, respectively. Thus, the Fcgr2b polymorphism is a key genetic element for not only Yaa-related but also Yaa-unrelated lupus.