Two-core LPS antibodies, the rabbit J5 polyclonal antiserum and the human anti-lipid A IgM mAb HA-1A, did not improve the survival of mice challenged with E. coli O111 or P. aeruginosa 3, or with the LPS extracted from them, and did not decrease the incidence of Shwartzman reactions in rabbits challenged with O111 LPS. In contrast, O side chain-specific rabbit antisera were protective in these models. The protection afforded by O side chain-specific antisera against endotoxin lethality was associated with decreased LPS-induced serum TNF and IL-6 levels, whereas core LPS antibodies had no effect on TNF or IL-6 levels. The absence of reduction of LPS-induced cytokines levels by core LPS antibodies suggests that these antibodies are not able to prevent the interactions between LPS and target cells.