The fragmentation patterns of synthetic Escherichia coli-type lipid A in plasma desorption mass spectrometry (PDMS) in both negative- and positive-ion modes were determined. Negative-ion spectra gave signals for the main diphosphorylated (intact) molecular species in their native proportions. Intact and alkaline-treated lipid A in this mode gave, for the glucosamine I moiety, easily identified signals that have not been previously reported in PDMS, These spectra gave enough information to localize the fatty acids. The procedure was verified with relatively homogeneous lipids A prepared from Salmonella minnesota R595 and Neisseria meningitidis lipopolysaccharides, and then applied to the previously unstudied Yersinia entercolitica O:11,24 lipid A to obtain the localization of its fatty acids. The possibility of obtaining this much information from two negative-ion spectra was attributed to the method, described earlier, of preparing the samples.In the positive-ion mode, about half of the E. coli ions containing diglucosamine appeared as monodephosphorylated species and/or as Na adducts. The intact glucosamine II moiety and its fragment ions gave signals none of which were Na adducts. With lipids A prepared from S. minnesota, N. meningitidis, and Y. enterocolitica, similar fragmentation patterns were observed, For lipid A structure determination, the positive-ion mode could play a confirmatory role, The above results and some of those reported by others were compared.