Endotoxins of Escherichia coli, Salmonella abortus equi and Serratia marcescens were examined using freeze-etch electronmicroscopy and light scattering techniques, with emphasis on their aggregation state. These endotoxins show distinct features of particle size and mass and they may be affected by the presence or absence of Ca2+ ions. No important differences were observed in the particle characteristics of the respective endotoxins in various infusion fluids.