The characteristics of hypotension induced in rabbits by intravenous injection of viable Salmonella typhosa O901 organisms were studied and found to be a function of the number and age of the bacterial cells. Effective neutralization of the blood pressure lowering was achieved by immunization with homlogous organisms as well as heterologous endotoxins and a detoxified derivative. In addition, native endotoxins derived from a number of different genera of gram-negative bacilli, as well as lipopolysaccharides deficient in either the polysaccharide or lipid components, were tested for their ability to induce hypotension in rabbits and tolerance to the lowering of blood pressure. Hypotension was elicited by intravenous injection of all native endotoxins as well as polysaccharide-deficient endotoxins, but was absent in preparations from which the lipid was removed. On the other hand, protection against the hypotension effect could be induced after injection of either the lipid- or polysaccharide-deficient derivatives.