The ability of 19 strains of Bacteroides fragilis to adhere to buccal epithelial cells (BEC) and to the human intestinal cell line HT-29 Clone 19A, and to agglutinate rabbit erythrocytes was compared. Adhesion to BEC was poor compared with that to the cell line. Adhesion to the latter was high for 21% of the strains, moderate for 37% and poor for 42%. Only 53% of the strains agglutinated rabbit red blood cells and only strain A459 did so strongly. Haemagglutination and adhesion of B. fragilis strain A459 were inhibited by sodium periodate, but not by proteases, heat or carbohydrates. These properties were not affected by protease which removed surface appendages. Periodate treatment did not remove the fimbriae or ruthenium red-staining layer, although the capsule was lost. This suggests that carbohydrate residues on the cell surface, possibly forming part of the capsule, are involved in adhesion and haemagglutination by this strain.