A particulate enzyme system from Phaseolus aureus seedlings catalyzes the synthesis of alkali insoluble polysaccharide material from UDP-d-glucose. 80 to 90% of the d-glucose units are joined by beta-1,4 linkages, the remainder being combined by beta-1,3 linkages. It is not known whether the material is a single polysaccharide or a mixture.With a substrate concentration of 4 x 10(-3)m the enzyme system catalyzes the formation of alkali insoluble polysaccharide from UDP-d-glucose at a rate 23 times greater than that from GDP-d-glucose. The modifications introduced also increase the formation of cellulose from GDP-d-glucose to 3 times the rate previously reported.None of the glycosyl nucleotides containing bases other than guanine or uracil (adenine, cytosine, thymine) served as substrate for the production of alkali insoluble polysaccharide with this enzyme system. dUDP-d-glucose could also not serve as substrate.