Both monoclonal antibody (MAb) and polyclonal antibody (PAb) directed against the shared core/lipid A region of lipopolysaccharide (LPS) (endotoxin) provide protection during experimental gram-negative bacterial sepsis. Although these preparations have not been compared, clinical trials administering either preparation to septic patients have been instituted. The core/lipid A region of LPS represents an antigenic domain common to many, if not all, gram-negative microbes, and thus represents an ideal target site for antibody binding. We sought to determine (1) the protective capacity of similarly reactive IgG anti-core LPS/lipid A MAbs and PAbs, (2) whether the timing of administration was important, and (3) whether either would act additively with antimicrobial agents. Antibody was administered intravenously to outbred mice, and Escherichia coli 0111:B4 was then administered intravenously or intraperitoneally with hemoglobin. Monoclonal antibodies and PAbs were equally protective, and protection was maximized by pretreatment, although the effect extended to four hours after bacterial challenge. Both MAbs and PAbs acted in concert with gentamicin hydrochloride to further reduce lethality. We concluded that MAbs and PAbs were equally protective and that clinical utility may eventually be dictated by ease and cost of antibody production.