A good correlation was found to exist between the serologically determined presence of enterobacterial common antigen (ECA) and the amount of the rare sugar constituent D-mannosaminuronic acid. Strains classified by serological techniques as ECA+, ECA-, and ECAtrace were found to possess the expected amounts of mannosaminuronic acid in the ECA-enriched phenol-soluble PL-L fractions. This correlation provides strong evidence on the identity of the mannosaminuronic acid-glucosamine polymer with the ECA as defined by Kunin (J. Exp. Med. 118:565-586, 1963).