A purified lipid A from Rhizobium leguminosarum biovar phaseoli CE3, as well as analogs and derivatives of the purified lipid A are provided. Compositions containing heterogenous mixtures of lipid A from R. leguminosarum bv. phaseoli CE3 are also provided. The lipid A provided herein can be combined with a pharmaceutically acceptable carrier. Methods of stimulating the immune system in a subject, treating or preventing toxic shock in a subject, and methods of treating and preventing a lipopolysaccharide mediated disorder in a subject using the novel lipid A are provided.