A simple physical-chemical method for estimating the lipopolysaccharide (LPS, i.e. endotoxin) content of meningococcal polysaccharide vaccines is described. The low molecular weight LPS was first separated from the high molecular weight polysaccharide by SDS-polyacrylamide gel electrophoresis. The LPS was then selectively visualized by a highly sensitive silver stain which can detect 5 ng of meningococcal LPS. The method can detect concentrations of LPS in meningococcal polysaccharides as low as 0.01%, and was used to measure semiquantitatively the LPS in bulk powders of meningococcal polysaccharides and in vaccines in final containers. The LPS in the bulk powders and in the vaccines was also assayed by the gelation of Limulus polyphemus ameobocyte lysate and the results of the two methods were in agreement.