Lipopolysaccharide from a smooth strain of Salmonella minnesota was fractionated into two major fractions and one intermediate fraction by using sodium dodecylsulfate-polyacrylamide gel electrophoresis. On the basis of the study by Hitchcock and Brown, it was deduced that the top fraction was mainly long O-side chain LPS and the bottom fraction was O-side chain-less LPS. The middle fraction was a mixture of both short O-side chain LPS and O-side chain-less LPS. The antigenic properties and biological activities were not altered in this fractionation procedure. Comparison of the biological activities of the top fraction with those of the bottom fraction revealed that the bottom fraction had higher activity in polyclonal B-cell activation and spleen-swelling effect and that there was no significant difference in adjuvant activity, ability to render macrophages cytotoxic, induction of colony-stimulating factor and the ability to induce the Schwartzmann reaction. It was suggested that O-side chain makes no contribution to the latter biological activities including adjuvant activity of S. minnesota LPS.