There is provided by this invention a novel method of preparing Gram-negative bacterial vaccines. The method comprises providing a concentrated Gram-negative bacterial antigenic preparation,adsorbing the preparation with a mineral carrier capable of binding free-endotoxin in the antigenic preparation in an amount effective to produce optimal binding of endotoxin and antigen and diluting the adsorbed preparation for use in a vaccine. Also provided by this invention is a vaccine produced by the method of this invention. Also provided by this invention is a Gram-negative bacterial vaccine wherein the improvement comprises a concentration of mineral carrier in the vaccine which is less than 5.0% v/v. Also provided by this invention is a Gram-negative bacterial vaccine comprising a mineral carrier wherein the amount of the mineral carrier in the vaccine has been predetermined by the method of this invention. Also provided by this invention is a method of vaccinating an animal against Gram-negative bacterial infections comprising administering to the animal an effective amount of a vaccine of this invention.