The aggregates produced by mixing saliva with cells of several strains of Streptococcus mutans, unlike those of Streptococcus mitis and Streptococcus sanguis, are extremely fragile. Aggregation of S. mutans cells can be followed spectrophotometrically but cannot be detected, after mechanical agitation, in an electronic particle counter. The salivary aggregating factor for a strain of S. mutans is shown to be different from those for S.mitis and S. sanguis. These findings are consistent with the poor adhesion of cells of S. mutans to saliva-treated hydroxyapatite and the relatively low incidence of S. mutans in dental plaque.