Dextran‐mediated agglutination has been both proposed and opposed as a mechanism for cohesive interactions between Actinomyces viscosus and streptococci in dental plaques. In their Studies, opponents of this mechanism used laboratory strains subcultured in vitro for unknown periods, whereas its proponents had previously shown that laboratory passage of Actinomyces strains weakened this trait. To clarify this issue, eighty‐eight strains of A. viscosus and Actinomyces naeslundii freshly isolated from the human oral cavity and hardly subcultured in vitro, were studied for enhancement of agglutination by commercially obtained polysaccharides including dextran, inulin, agarose, and starch and polysaccharides isolated from sucrose‐grown Streptococcus mutans, Streptococcus sanguis, and Streptococcus salivarius. Few of the strains exhibited higher agglutination scores in buffer containing any of the various polysaccharides than in control buffer free of the polysaccharides. Infrequent polysaccharide agglutinability of these human isolates, even by polysaccharides likely to be encountered in dental plaques, casts doubt on this explanation for Actinomyces’cohesive interactions in plaque formation.