A sensitive silver stain for detecting bacterial lipopolysaccharides in polyacrylamide gels is developed by modifying the silver-staining method used for proteins (cf. R. C. Switzer III, C. R. Merril, and S. Shifrin, Anal. Biochem.98, 231–237 (1979). Lipopolysaccharides are analyzed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate followed by visualization with either the modified silver stain or periodic acid-Schiff stain. The lipopolysaccharides are stained dark brown by the silver stain. The silver stain is 500 times more sensitive than the periodic acid-Schiff stain and can detect less than 5 ng of rough type lipopolysaccharides. Analyses of 5μg of smooth-type lipopolysaccharides from Salmonella typhimurium and Escherichia coli O111: B4 show each to have 30–40 components of different molecular weights. The use of a lipopolysaccharide having a known structure and variable numbers of repeating units in the O side chain, such as one of the two lipopolysaccharides mentioned above, as molecular weight markers is proposed for the estimation of the molecular weights of other lipopolysaccharides or their components. The lipopolysaccharides can also be stained grayish green, but become grayish blue with a heavy sample load, using a silver-based color-staining method (D. W. Sammons, L. D. Adams, and E. E. Nishizawa, Electrophoresis2, 135–141 (1981)).