A highly purified monophosphoryl lipid A, TLC-3 fraction obtained from the lipopolysaccharides of the heptoseless mutant Salmonella typhimurium G30/C21 was converted to the dimethyl pentatrimethylsilyl derivative and analyzed by proton NMR spectroscopy at 400 MHz. Substantial downfield shifts of the resonances for protons at the 3- and 3'-carbons of the glucosamine disaccharide to 5.06 and 5.15 ppm, respectively, occurred from the normal range of 3.5-4.1 ppm, indicating that these two positions on the sugar rings were acylated. Significant downfield shift of the resonances for protons at the 4- and 6'-carbons did not occur, indicating the absence of acyl groups at these two positions. Since positive ion fast atom bombardment mass spectrometry previously established the presence of hydroxymyristoyl and myristoxymyristoyl esters at the reducing end and distal subunits, respectively, these acyl groups must be attached to the oxygen of the corresponding 3- and 3'-carbons of lipid A. With these results, we can now describe the complete structure of the monophosphoryl lipid A, TLC-3 from S. typhimurium.