A vaccine, effective in inducing the production of antibodies with which to immunize a second subject passively against infection by Gram-negative bacteria and LPS-mediated pathology, comprises a non-covalent polyvalent complex formed between purified, detoxified LPS derived from E. coli and purified outer membrane protein derived from N. meningitidis. The same vaccine will also actively immunize a host subject against Gram-negative bacterial infections and LPS-mediated pathology. Meningococcal infections are included among those Gram-negative bacterial infections protected against by the vaccine.