A particulate enzyme from cotton fibers transfers mannose from GDP- 14C-mannose and glucose from UDP- 14C-glucose to a phosphate-containing lipid which is present in these membrane particles. Both glycolipids are retained on DEAE-cellulose and both have identical R f values in three different solvent systems. The sugar moieties of these lipids are very acid labile so that 50% of the sugar is released in 0.001 – 0.003 N HCl at 100° in 5 minutes. Both lipids are able to donate their sugar moieties to the respective nucleoside diphosphate to form sugar nucleotides (mannolipid to GDP to form GDP-mannose; glucolipid to UDP to form UDP-glucose). Thus, these lipids are similar in many respects to the mannosyl-phosphoryl-polyprenols synthesized from other sources. During studies on the biosynthesis of cell-wall polysaccharides by particulate extracts of cotton fibers, it was observed that radioactive mannose from GDP- 14C-mannose was incorporated into chloroform:methanol (C:M) soluble products. Since the kinetics of incorporation suggested that the lipid might be turning over, we examined the properties of the mannolipid and found that it was quite similar to the mannosyl-phosphoryl-undecaprenol isolated by Scher et al (1) and shown to be an intermediate in mannan biosynthesis in Micrococci (2). Furthermore, the 14C-mannolipid can donate its mannosyl moiety to GDP to form GDP- 14C-mannose. The particulate enzyme from cotton fibers also transfers glucose from UDP- 14C-glucose to a lipid which has similar properties to those of the mannolipid. The lipid moieties of these glycolipids have not yet been identified but both of these compounds have similar R f values to those reported for mannosyl-phosphoryl-polyprenols from bacteria (2) and mammalian sources (3).