Quantitative studies have demonstrated that a purified lipopolysaccharide (endotoxin) derived from the O-901 strain of Salmonella typhosa markedly enhanced the antibody response of rabbits when given separately or in conjunction with protein antigens. The augmentation of antibody levels varied from 2- to 40-fold with the number of injections, the dosage of antigen and endotoxin, and the route of administration. This antibody-enhancing property was found to be common to a broad group of endotoxins from Gram-negative bacilli and was not restricted to the lipopolysaccharide derived from S. typhosa.

Factors affecting this enhancement were investigated, and data are presented which indicate that host susceptibility to endotoxin is a prerequisite for elevation of antibody levels; the intact lipopolysaccharide, as isolated, might not be essential for this activity; and the rate of clearance of antigen from the circulation of rabbits was accelerated when endotoxin was given in conjunction with protein.

The data obtained are discussed in relation to postulated mechanisms on the antibody-enhancing action of endotoxin.

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