A neutral glucorhamnan has been isolated from the lipopolysaccharide of the O10 reference strain (C.D.C. 1287-54) of Serratia marcescens. By means of n.m.r. spectroscopy, methylation analysis, and degradative studies, the polymer (the putative O-specific antigen) was found to have the branched, pentasaccharide repeating-unit shown. (formula; see text).