Lichenin, the poly-β-D-glucan ofCetrariaislandica(Iceland moss), is found by enzymic degradation to differ in fine structure from the poly-β-D-glucans of cereal grains. Enzymolysis has been carried out with a cellulase and a laminarinase preparation, the former yielding mainlyO-β-D-glucopyranosyl-(1 → 3)-O-β-D-glucopyranosyl-(1 → 4)-α-D-glucose, and the latter mainlyO-β-D-glucopyranosyl-(1 → 4)-O-β-D-glucopyranosyl-(1 → 3)-α-D-glucose. Di-and tetra-saccharides are produced in small proportions. Steric aspects of these enzymic degradations are discussed. The basis constitution of lichenin is represented by a tetrameric unit in which two adjacent (1 → 4) linkages alternate with an isolated (1 → 3) linkage; occasionally four consecutive monomers are linked by (1 → 4) bonds. The glucans of cereal origin differ mainly in possessing a higher proportion of the latter structural sequence. Despite their close chemical similarity to cellulose, all of these glucans are soluble, a property that appears to be related to the even distribution of β-(1 → 3) bonds along the chains, with a resulting disruption of linear orientation effects.