PARTICULATE enzyme preparations from bacteria1 and plants2,3 have been reported to catalyse the biosynthesis of high molecular weight β-1,4-linked glucan, chiefly cellulose, from either UDP-glucose or GDP-glucose. Partial hydrolysis of the polysaccharide formed from UDP-glucose, however, indicates the presence of β-1,3-linked glucose moieties4,5. Moreover, GDP-glucose is not found as a constituent of plant tissues that are actively forming cellulose6,7; and the enzyme system that utilizes GDP-glucose increases and decreases in catalytic activity as a function of the growth stage of the plant2,8, and these fluctuations do not coincide with the production of cellulose9. We now suggest that GDP-glucose is the in vivo precursor of a glucomannan rather than cellulose.