The objectives were to determine the degree of Actinomyces agglutinating activity in human saliva and to begin characterizing the agglutination mechanism. Agglutination titres of whole saliva collected from adults and 6-yr-old children were compared. Titres for A. naeslundii were always higher than for A. viscosus. The mean A. naeslundii titre for the adults' and children's samples were equivalent. The children had a slightly lower mean titre than the adults for A. viscosus. No correlation was found between IgA concentrations and agglutination titre. Agglutinating activity was partially impaired by incubation with anti-IgA serum. Activity in submandibular/sublingual saliva was resistant to heat at 56 degrees C but sensitive to boiling. Boiling the bacteria had no effect. In sugar inhibition tests, only galactosides (beta-Gal) and glucosamine (for A. viscosus) affected Actinomyces agglutination but impairment was only temporary. Agglutinating activity was diminished by incubating saliva with hydroxyapatite. Thus, Actinomyces agglutinins 1) are probably distinct from IgA but may complex with it; 2) may include both beta-Gal and higher affinity sites; and 3) may contribute to salivary pellicle.