Martin, William J. (University of Utah, Salt Lake City), and Stanley Marcus. Detoxified bacterial endotoxins. II. Preparation and biological properties of chemically modified crude endotoxins from Salmonella typhimurium. J. Bacteriol. 91:1750-1758. 1966.-Chemical modification of a crude endotoxin prepared by the Roschka-Edwards (RE) procedure from a strain of Salmonella typhimurium yielded products which were nontoxic for mice and had reduced fever effects in rabbits. A reduction in rabbit pyrogenicity of approximately 100 times was noted with a potassium periodate-treated RE preparation when compared with the parent RE preparation. Measured in a similar fashion, pyrogenicity of a potassium methylate-treated RE preparation was reduced by a factor of 10 while pyrogenicity of a boron trifluoride RE preparation was unchanged. All of these endotoxoids, including the parent RE preparation, showed little toxicity for mice. Immunogenicity was determined in mice by comparing Boivin, RE, and endotoxoid preparations with a heat-killed, phenol-preserved (HP) vaccine prepared from the same strain of S. typhimurium. Employing a 10 ld(50) challenge, the protective immunogenicity of the respective vaccines was determined by active immunized mouse protection tests. Although two 100 mug immunizing doses of the Boivin, RE, and the respective endotoxoid preparations varied in mouse protection (potassium methylate RE > Boivin > RE > boron trifluoride RE > potassium periodate RE), it was evident that, with the exception of the potassium methylate preparation, the HP vaccine yielded greatest protection against the 10 ld(50) challenge with S. typhimurium. Further mouse protection experiments suggested that the minimal immunogenic dose of the potassium methylate RE vaccine preparation was approximately 50 mug. These data indicated an approximate fivefold difference between the minimal pyrogenic dose (10 mug) and the minimal immunogenic dose (50 mug). These findings further suggest that potassium methylate RE vaccine preparations should be considered in the search for less toxic enteric fever vaccines.