Evidence for the presence of undecaprenol in the unsaponifiable lipid of Lactobacillus plantarum (N.C.I.B. 6376) is presented. Characterization of the compound was based mainly on mass, i.r. and n.m.r. spectrometry. The prenol was isolated at a concentration of 40μg/g wet wt. of bacteria and contained over 90% (1.0–5.4% of the dose) of the 14C present in the unsaponifiable lipid after incubation of the bacteria with [2-14C]mevalonate. N.m.r. spectrometry indicated the presence of two internal trans-, one α-cis- and seven internal cis-isoprene residues per molecule. The 3H/14C ratios of the prenol after incubation of the bacteria with [2-14C,(4R)-4-3H1]- and [2-14C,(4S)-4-3H1]-mevalonate were in agreement with this stereochemistry. There was no evidence of saturated isoprene residues in the molecule. The undecaprenol appeared to be accompanied by much smaller quantities of decaprenol and nonaprenol.