In order to produce antiserum with broad protection against different Gramnegative bacteria we immunized rabbits with a UDP-galactose-deficient mutant (J5) of Escherichia coli O111 that is unencumbered by O antigen since it cannot incorporate galactose into core LPS. Without O side chains, core is accessible for producing antiserum to a wide range of bacteria with similar cores. Such antiserum has previously been shown to prevent unrelated endotoxins from producing dermal and generalized Shwartzman reactions, and death. The current study reports the results of treating bacteremia with this antiserum. Bacteremia was produced in rabbits made granulocytopenic with nitrogen mustard and fed either Klebsiella pneumoniae, E. coli O17 (multiply antibiotic-resistant), or E. coli O4. Since most rabbits are coliform-free, the organisms colonized the gut and overwhelming bacteremia occurred as granulocytes disappeared. When nonimmune serum was given intravenously (i.v.) at the onset of bacteremia with any of these three organisms, only 3.1% to 5.7% survived. In contrast, survival rates in animals given antiserum to the J5 mutant were 33.3% from E. coli O4, 40% from Klebsiella, and 69.2% from the multiply antibiotic-resistant E. coli O17 (combined p < 0.0005). Antiserum to the parent E. coli O111, (with O antigen intact) provided no significant protection. The protection against death from E. coli O17 by J5 antiserum was greater than that obtained from gentamicin (50%), the only antibiotic to which this strain was sensitive. Rabbits given J5 antiserum cleared labeled bacteria more rapidly than those given nonimmune sera. Antiserum to the J5 mutant is thought to prevent lethal bacteremia from unrelated Gram-negative bacteria by acting as both antitoxin and opsonin.