Corwin, L. M. (Walter Reed Army Medical Center, Washington, D.C.), and W. E. Farrar, Jr. Nature of the endotoxin-inactivating principle in guinea-pig liver. J. Bacteriol. 87:832–837. 1964.—Guinea-pig liver preparations inactivate Serratia marcescens endotoxin as assayed in chick embryo. The activity is optimal at pH 6.5 to 7.0 and 8.5 to 9.0. Mitochondria and the supernatant fraction containing microsomes possess activity. Mitochondria are only active at the acid pH optimum. The activity of acetone powder extracts of mitochondria is enhanced by adenosine triphosphate and nicotinamide adenine dinucleotide, whereas the mitochondria themselves are also activated by malate. It was concluded that the enzymes which inactivate endotoxin involve fatty acid activation and oxidation. Such a finding suggests that the lipid moiety of endotoxin is required for toxicity.