Reactions between selected strains of oral streptococci and saliva were studied spectrophotometrically. The curves of absorbance (700 nm) versus those of time were sigmoidal for aggregating saliva-bacteria mixtures. Whtn the logarithm of the ratio of the change in absorbance to the remaining absorbance was plotted against the reaction time, straight lines were obtained. At a fixed concentration of bacteria, the slope of these lines for a series of dilutions of a selected saliva sample was a parabolic function of the volume of saliva in the reaction mixtures. A model for the aggregation reaction was developed which explained the shape of these parabolic curves. The parameters of the model can be calculated from the experimental data and can be used to estimate the concentration of aggregation-inducing substances in undiluted saliva. A method for calculating the specific rate constant for the binding of free cells to aggregates was developed. This rate constant is a characteristic feature of a given aggregation reaction and can, in principle, be used to distinguish between aggregation-inducing substances of different molecular structures.