The structure of the O-specific side-chains of the Escherichia coli O 10 lipopolysaccharide has been investigated. Methylation analysis, n.m.r. spectroscopy, and various specific degradations were the principal methods used. It is concluded that the polysaccharide is composed of pentasaccharide repeating-units having the following structure, in which D-Fuc4NAcyl is 4-amino-4,6-dideoxy-D-galactose, acylated with acetic acid (60%) or D-3-hydroxybutyric acid (40%). (Formula: see text).