All of these experiments demonstrate that despite many microbiologic differences between Pseudomonas and the enteric rods, there is a close immunologic relationship between their endotoxin cores. These studies also show that by genetic manipulation we can unmask a protective antigen for preparing antiserum with a broad range of activity, even in the face of neutropenia. Production of potent human J5 antiserum is a safe, simple procedure, making it possible to consider endotoxin core antibody treatment of all Gram negative bacteremia in patients regardless of the causative organism.