Summary: The effects of physical and chemical agents on the biological properties of purified enterobacterial lipopolysaccharides have been studied. The following observations have been made: The untreated lipopolysaccharides are markedly effective as antibody neutralizing antigens. They possess a high degree of toxicity and pyrogenicity, but they have only a limited erythrocyte-modifying capacity.Treatment of these lipopolysaccharides with heat or with sodium hydroxide for short periods of time results in an increase of their capacity to modify erythrocytes without alteration of their antigenicity (antibody-neutralizing potency) toxicity or pyrogenicity. Treatment with sodium hydroxide for longer intervals does not affect their antibody-neutralizing and erythrocyte-modifying capacity, but results in a marked loss of toxicity and pyrogenicity.Oxidation of the Escherichia coli lipopolysaccharide with periodate yields a compound of low toxicity and marked pyrogenicity. The capacity of this substance to neutralize antibody is almost completely abolished.A degraded, lipid-free polysaccharide component obtained from Salmonella abortus equi lipopolysaccharide exhibited reduced antibodycombining capacity when compared with the original product; treatment with heat and sodium hydroxide fails to produce any erythrocyte-modifying capacity.Red blood cells which have been modified with heated or sodium hydroxide treated lipopolysaccharides specifically absorb homologous bacterial antibodies. Erythrocytes which have been in contact with equivalent amounts of untreated antigens do not.The results of this study indicate that the various biological activities of bacterial lipopolysaccharides are affected differentially by different physical and chemical agents.The significance of the data has been discussed with particular reference to the relationship of chemical and physical-chemical characteristics of enterobacterial lipopolysaccharides and their components to the various biological activities of these compounds.