Abstract: A lipopolysaccharide characterized by a polysaccharide portion composed of D-arabinose and D-mannose in a 1:3/4 ration and 37 to 47% of a fatty acid portion having 14-19 carbon atoms bonded to the polysaccharide through an ester linkage. This lipopolysaccharide has physiological activities such as antitumor activity, immunizing activity, cell juvenescent activity, phagocyte activating activity, and infection preventing activity.This lipopolysaccharide is prepared by culturing a Mycobacterium or a Propionibacterium and extracting the lipopolysaccharide from the culture with a non-ionic surface active agent and purifying the extract with a molecular sieve.