The relationship between oral implantation of Streptococcus mutans IB1600 (serotype c) and Streptococcus sobrinus OMZ65 (serotype g), the aggregating activity of saliva, and its influence on the adherence of these bacterial strains in vitro was examined in seven human subjects. All the saliva samples aggregated strain IB1600 but not strain OMZ65 cells. Whole saliva from subjects with low levels of infection by Strep. mutans aggregated strain IB1600 to a greater degree than did whole saliva from those who were readily infected. Whole saliva from subjects most resistant to infection supported the adsorption of the highest number of either strain IB1600 or OMZ65 to hydroxyapatite surfaces. Thus the ability of whole saliva to aggregate Strep. mutans may influence the ability of these microorganisms to infect the mouth.