Cells of several species of oral microorganisms have been shown, in earlier studies, to be aggregated by saliva. In the present study some of the basic properties of the aggregation system are examined. The observation is made that the saliva-induced aggregates of Streptococcus sanguis and S. mitis can be dissociated to stable particles which consist of about 100 cells and have a median diameter of about 4.5 mum. It is proposed that these are subunits, or core aggregates, of the large primary aggregates. Counts of the core aggregates can be taken as a precise and accurate measure of aggregation. Experiments based on this procedure show that the aggregation of S. sanguis is maximal at 10 C and at 1 meq of Ca(2+) ions per liter and is not affected by a change in pH between 3.9 and 8.7 or by a change in the phase of growth of the microorganisms. Core aggregates diminish in number with prolonged incubation, suggesting that the aggregating factors break down with time. Formalinized cells yield stable aggregates. However, with Formalinized cell aggregation is maximal between 20 and 30 C and proceeds in the absence of calcium ions. Evidence is presented that whole saliva contains separate aggregating factors for S. sanguis and S. mitis. The factors differ in their affinity for intact cells and for hydroxyapatite and differ in their stability to dialysis. These findings suggest that many different aggregating factors exist in saliva, each of which may be capable of interacting with cells of one or several bacterial species.