A newly isolated serogroup of Neisseria meningitidis (serogroup L), obtained from contacts of a patient with meningococcal meningitis, elaborates a structurally unique, capsular polysaccharide. The polysaccharide contains only 2-acetamido-2-deoxy-D-glucosyl and phosphate constituents in the molar ratio of 3:1, and is composed of the following repeating unit.