Chemically synthesized (1→3)- a- d-glucans, dextrans from Leuconostoc spp., and glucans from Streptococcus sobrinus strains were examined by enzymic degradation, methylation analysis, and n.m.r. spectroscopy. (1→3)- a- d-Glucans were hydrolyzed completely to d-glucose and nigerose by an endo-(1→3)- a- d-glucanase from Cladosporium resinae. The extent of hydrolysis of native Leuconostoc dextrans, in which by definition 50% or more of the linkages are (1→6), and of S. sobrinus mixed-linkage glucans, was limited, not only by their linkage compositions, but also by the arrangement of the (1→3) and (1→6) linkages and by accessibility to the enzyme. Estimated enzymic conversions of several water-insoluble glucans, which contained 11–63% of (1→3) linkages, agreed with the experimental values when calculated on the basis of a 4% content of branch points involved in the attachment of (1→6)-linked chains to the (1→3)-linked chains, and four resistant linkages for each such interchain branch-point. By contrast, the more-soluble Leuconostoc dextrans contained fewer (1→3) linkages (1–5%) and their complete susceptibility to enzymic hydrolysis indicated their peripheral location. Three soluble dextrans, synthesized by three different a- d-glucosyltransferases (GTF-S), isolated from culture filtrates of S. sobrinus, contained only small proportions of (1→3) linkages not involved in branching, and they were resistant to the endo-glucanase. The relationship for 37 glucans between their enzymic hydrolysis and the proportion of (1→3) linkages permits reliable and sensitive estimates of the sequences of (1→3) linkages.