Dextran‐mediated agglutination has been both proposed and opposed as a mechanism for cohesive interactions betweenActinomyces viscosusand streptococci in dental plaques. In their Studies, opponents of this mechanism used laboratory strains subculturedin vitrofor unknown periods, whereas its proponents had previously shown that laboratory passage ofActinomycesstrains weakened this trait. To clarify this issue, eighty‐eight strains ofA. viscosusandActinomyces naeslundiifreshly isolated from the human oral cavity and hardly subculturedin vitro, were studied for enhancement of agglutination by commercially obtained polysaccharides including dextran, inulin, agarose, and starch and polysaccharides isolated from sucrose‐grownStreptococcus mutans, Streptococcus sanguis, andStreptococcus 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 forActinomyces’cohesive interactions in plaque fo