The lipopolysaccharides (LPS) of strains of Neisseria gonorrhoeae grown in type 1 (T1) and 4 (T4) colony forms have been isolated. LPS from T4 colony type cells on mild hydrolysis gave a lipid A and a core oligosaccharide composed of 2-amino-2-deoxy-D-glucose, D-glucose, D-galactose, L-glycero-D-manno-heptose and 3-deoxy-D-manno-octulosonic acid that appeared to be common to all the strains examined. LPS from T1 colony type cells on mild hydrolysis gave a lipid A and high molecular weight O polysaccharides which showed considerable differences in glycose composition for each strain examined. In those strains examined, T4 cells appear to produce a common 'R' type LPS whereas T1 cells produce an 'S' type LPS with structurally different O polysaccharide structures which probably account for serologically differentiated strains of N. gonorrhoeae.