To provide a lipopolysaccharide (LPS) that have reduced phospholipid content and a method for producing it.

The method allows for the production of the LPS which can be used to produce a 3-O-deacylated monophosphoryl lipid A (3D-MLA) having at least about 20 mol% of the hexaacyl congener group, and comprises: growing a culture of deep rough mutant bacterial strain in a medium; harvesting cells from the culture; and extracting LPS from the cells. Also the method of extracting lipopolysaccharide (LPS) from a culture of deep rough mutant bacterial strain cells comprises: extracting the cells with a solution consisting essentially of an aliphatic alcohol and the balance water, thereby producing cells with reduced phospholipid content; and extracting the cells with reduced phospholipid content with a solution comprising chloroform and methanol, thereby yielding a solution of LPS. This method provides an LPS solution that has reduced phospholipid content and is produced by relatively simple and inexpensive process steps.