The rough lipopolysaccharide (LPS) from Erwinia carotovora was found to form several mu m diameter vesicles in aqueous solution, Spontaneous formation of spherical particles at ambient temperature was observed on addition of ethanol or a multivalent metal chloride to the solution. The LPS altered the gel-liquid crystalline phase transitions of the hydrocarbon chains in the temperature range 13-32 degrees C. The vesicle formation indicated a marked increase of phase transition enthalpy Delta H up to three times greater than that of the LPS dispersed in pure water. The thermotropic phase behavior of the LPS was affected by metal ions, ionic strength and pH of the solution, Protonation of the LPS in acidic solution caused the transition temperature t(m) and Delta H to increase, whereas deprotonation in alkaline solution led to the opposite effect, Multivalent metal ions such as Mg2+ and Ca2+ raised the t(m) and increased the Delta H value, The LPS was organized to a greater degree in saline, acidic solution or divalent metal chloride solution than in pure water. The phase behavior varied according to the solvent conditions or metal chlorides that affect the phosphate and carboxyl groups, indicating that these charged moieties have an influence on the molecular organization within the membrane, Approximately two molar equivalents or more of Mg2+ ion induced a prominent thermotropic peak at approximate to 13 degrees C. This ionotropic phase property of the natural LPS is supposedly directed by cross-linkage through divalent metal ions and negatively charged moieties such as phosphate bound to the LPS molecule. The functions of the LPS membrane are thus susceptible to change according to the chemical and physical conditions of the environment.