A serological grouping scheme for Streptococcus sanguis has been developed. This scheme was based on an Ouchterlony and immunoelectrophoretic analysis of extracts of strains of S. sanguis. The number of antigens which could be identified in these extracts was one to five and were identified as a to e. Because of the wide distribution the a antigen was considered to be group specific whereas the other antigens were found less frequently and were considered to represent type specific antigens. Using this taxonomic scheme as a starting point, a number of properties, i.e., cell wall composition, salivary aggregation, lysogenic conversion, transformation and production of autolytic enzymes,will be examined to determine if a relationship exists between these properties and the antigenic structure of these cells. Because most or all of these properties involve polymers which are antigenic an immunochemical approach to the study has been adopted. It is hoped that these studies will enable us to determine if modulation (change in the antigenic structure) of dental plaque is regulated by continuous reinfection with new serotypes or whether an internal genetic mechanism among idiogenous flora is responsible for modulation. If specific serotypes are involved in plaque formation, then it might be possible to prepare specific immunological or chemical reagents which could inhibit activities or "block" polymers responsible for plaque development.