The coaggregation-defective (COG-) mutant Actinomyces viscosus T14V(PK455), which is unable to participate in lactose-sensitive adherence, and its COG+ parent were compared to structurally define the mutational loss of cell-associated lectin activity. Immunoelectrophoretic comparisons of crude extracts or isolated fimbriae from both strains demonstrated type 2 fimbriae (previously designated Ag2) in preparations obtained from the parent but none in those obtained from the mutant. This result was verified by the immunoelectron-microscopic identification of type 1 (previously designated Ag1 or VAl) and type 2 fimbriae on the parent organism but only type 1 fimbriae on the mutant. A comparison of the amounts of extractable fimbriae of each type and the capacity of the cells to bind 14C-labeled monoclonal antibodies specific for each fimbrial component showed that the mutational loss of type 2 fimbriae had no significant quantitative effect on fimbriation of the COG- mutant with type 1 structures. Cells of A. viscosus T14AV, a mutant with various adherence defects that include the COG- phenotype, displayed fimbriae of both types, but in greatly reduced amounts. Thus, the properties of mutant strain T14V(PK455) associated the lectin activity with type 2 fimbriae, whereas those of strain T14AV provided little insight into the mechanism of lactose-sensitive adherence. In addition, the precise nature of the cell surface modification displayed by strain T14V(PK455) provides clear evidence for the existence of distinct and independent fimbriae on A. viscosus T14V.