Hyper-elevated immune response of FcGRIIb−/− mice, a lupus model with an inhibitory-signaling defect, can become exhausted (less subsequent immune-response than the first response) with sequential lipopolysaccharide (LPS) stimulation. Endotoxin tolerance-related modifications of inflammatory response were investigated in FcGRIIb−/− mice in both an in vivo sepsis model and in vitro using cultured macrophages. Serum cytokine concentrations, after the second LPS injection (at 5-fold higher levels than the first dose), did not exceed the first dose levels in either FcGRIIb−/− or wild-type mice. These data indicated an endotoxin-tolerance response in both genetic backgrounds. However, the difference of cytokine levels between the first and second LPS injection was more prominent in FcGRIIb−/− mice. More importantly, CLP-induced sepsis after LPS-preconditioning (two separated doses of LPS administration) was more severe in FcGRIIb−/− mice (as measured by mortality rate, bacteria count in blood, serum cytokines, creatinine, and alanine transaminase). An attenuated response was demonstrated after two sequential LPS stimulations of bone-marrow-derived macrophages. Cytokine production was reduced and lower bacterial killing activity occurred with macrophages from FcGRIIb−/− mice relative to wild-type macrophages. Thus, there is a more prominent effect of endotoxin-tolerance in FcGRIIb−/− macrophages relative to wild-type. In conclusion, repeated-LPS administrations induced quantitatively greater endotoxin-tolerance responses in FcGRIIb−/− mice both in vivo and in vitro. Endotoxin-tolerance in vivo was associated with more severe sepsis, at least in part, due to macrophage-dysfunction.