Streptococcus mutans is a major etiological agent in dental caries. Salivary agglutinin is one of the main salivary components binding to S. mutans. To learn more about the interaction of salivary agglutinin with S. mutans, parotid, submandibular, sublingual and palatal saliva samples were incubated with S. mutans suspension. Both depleted saliva samples and bacterial extracts were analyzed by SDS-PAGE and immunoblotting. Salivary agglutinin was present in all types of glandular saliva and in all cases bound to S. mutans, also to PC337C, a P1 mutant of S. mutans. Agglutinin was separated by SDS-PAGE under reducing and non-reducing conditions and then transferred to nitrocellulose. Non-reduced agglutinin bound S. mutans, but reduced agglutinin did not. Adhesion of S. mutans to agglutinin-coated microplates was inhibited by amine-containing components, 1 M NaCl or KCl and EDTA. Adhesion decreased with decreasing pH with no adhesion below pH 5.0. These data suggest that calcium-dependent electrostatic interactions play a role in binding. By immunoblotting was demonstrated that blood group antigens and Lewis antigens were present on agglutinin. Synthetic blood group antigens and Lewis antigens covalently coupled to polyacrylamide were tested for binding to S. mutans. Only Le(a)(Gal beta 1,3(Fuc alpha 1,4)GlcNAc) bound to S. mutans, whereas the blood group antigens Le(b), Le(x), Le(y), H1, H2, A, B and sialylated Le(a) did not. Lea without galactose (Fuc alpha 1,4GlcNAc) still bound to S. mutans, but Le(a) without fucose (Gal beta 1,3GlcNAc) did not. Binding of agglutinin to S. mutans was not inhibited by Le(a). In conclusion, S. mutans can bind to Le(a) carbohydrate epitopes in which the fucose is an essential residue. Le(a) carbohydrate epitopes are present on salivary agglutinin but play no major role in binding.