THE action of the polysaccharide-synthesizing enzyme dextransucrase has been described previously in terms of pH1 and the effect of various alternate receptors2,3. We have now examined the dependence of rate of reaction on the concentration of enzyme. The results are interpreted on the basis of adsorption kinetics4, where the substrate (sucrose) and acceptor are adsorbed simultaneously on adjacent sites of the enzyme. This is followed by glucosyl transfer from sucrose to acceptor. No reaction occurs between bound sucrose and acceptor in solution, and there is no separate hydrolytic step prior to the transfer of the glucosyl group.