To study the effect of age and antigenic priming on the development of gingivitis, 33 healthy rats were placed in contact with Streptococcus mutans, Actinomyces viscosus, Fusobacterium nucleatum, and Bacteroides gingivalis. On days 0, 3, 7, and 14 after inoculation, the gingival condition was judged clinically and histologically, and serum antibody titers against the bacteria were measured. The rats were divided into three groups: 1 month old, 3 months old, and 3 months old immunized. None of the young rats developed gingivitis during the experiment, whereas half of the adult and all of the adult immunized rats bled on probing on days 7 and 14. In general, antibody titers against the bacteria were low in young rats, moderate in adult rats, and high in adult immunized rats. These results indicate that adult rats react stronger to plaque antigens than young rats and that previous contact with the antigens increases the reaction.