The fatty acid profiles of endotoxin isolated from a number of bacterial species, including three preparations of Enterobacter agglomerans, two of Escherichia coli, Salmonella typhimurium, Salmonella abortus equi, and Serratia marcesens, and one of Pseudomonas syringae, were obtained. The isolated endotoxins were hydrolyzed with alcoholic NaOH, acidified, and the fatty acids extracted with petroleum ether. The dried extract was derivatized with phenacyl bromide and the phenacyl ester was chromatographed on a high performance liquid chromatograph. The fatty acid composition varies for each bacterial species and even for different preparations of endotoxin from the same bacterial species (E. agglomerans). The biological activities of two preparations of E. agglomerans have been obtained by various workers and some of these are included.