Nowotny, Anna M. (City of Hope Medical Center, Duarte, Calif.), Scott Thomas, Olga S. Duron, and Alois Nowotny. Relation of structure to function in bacterial O antigens. I. Isolation methods. J. Bacteriol. 85:418–426. 1963.—Six isolation methods, four previously described and two reported herein, were compared for the extraction of endotoxic O antigens from three different enterobacteria: Serratia marcescens Bizio, Salmonella typhosa O901, and Escherichia coli K-12 lambda. Large variations were observed in the toxin yield, in the serologically reactive material yield, and in the nucleic acid-releasing abilities of the six methods when applied to the different bacterial strains. Chemical analyses of the toxic O antigens included quantitative determination of the amino acids, long- and short-chain carboxylic acids, and carbohydrates. Comparisons of the nucleic acid content were also made. Although the chemical composition of the preparations was quite different, no correlation could be found between the percentage of the above components and the lethal toxicity or the serological reactivity. It was concluded that the structural elements or properties responsible for these biological activities cannot be detected by gross chemical analyses.