Glucans synthesized from sucrose by cell-free glucosyltransferase of Streptococcus mutans AHT were separated into water-insoluble (ISG) and water-soluble (SG) fractions by centrifugation. The isolated ISG comprised coalescent clumps of two fibrillar components. One component consisted of two protofibrils approx. 4nm thick in a parallel arrangement and bearing short fluffy protrusions on the side surface. The other component of ISG, and the isolated SG also, was a single-stranded fibril, about 2 nm thick, usually in fine meshworks and sometimes thick clumps. Dextranase treatment of ISG removed the single-stranded fibrils and the fluffy peripheral protrusions of the double strands. The results indicate that ISG is a mixture of fibrillar components, and suggest that glucans produced by Strep. mutans AHT can be classified into two different types: an inherently water-insoluble, dextranase-insensitive, double-stranded fibril having dextranase-sensitive peripheral fluffy protrusions, and a separate dextranase-sensitive single-stranded fibril, present free in aqueous solution or in coherent association with water-insoluble double strands. The double strands of ISG were associated tangentially with the fuzzy coat of Strep. mutans AHT cells via their peripheral protrusions, whereas the single strands of ISG and SG attached perpendicularly. The residual-core double strands of dextranase-treated ISG did not attach to the fuzzy coat. ISG and SG did not bind to heat-treated cells, which showed no detectable morphological alteration of the cell surface envelope, nor did they bind to trypsin-treated cells lacking the bulk of the fuzzy coat. Recultivated in glucose broth, trypsinized cells recovered glucan-binding activity before detectable reconstruction of the fuzzy coat, to which the two types of fibrillar glucans attached in their characteristic manners, thus confirming that α-l,6-linked glucose residues of ISG and SG and a heat-labile, proteinaceous component of the cell-surface fuzzy coat participate in the glucan-induced agglutination of Strep. mutans cells.