A ctinobacillus actinomycetemcomitans is a Gram negative bacterium which has been implicated in the etiology of Juvenile Periodontitis disease. The association between Localized Juvenile Periodontitis and Actinobacillus actinomycetemcomitans infection is indicated by results of a study in which 96,5% Localized Juvenile Periodontitis subjects were found to harbor the organism in subgingival plaque. Actinobacillus actinomycetemcomitans is able to evade host defense mechanism by its ability to produce a number of virulence factors. It has been shown that patients with Localized Juvenile Periodontitis display a defect in their PMN. Localized Juvenile periodontitis includes advanced destructive lession of periodontal tissue in children and adolescents. The localized form can changes become to generalized, because the localized nature of Juvenile Periodontitis relies on adequate immune responses and bacterial antagonism. There are three serotypes of Actinobacillus actinomycetemcomitans which termed serotypes a, b, and c. Each Localized Juvenile Periodontitis patient is infected with only a single serotype of Actinobacillus actinomycetemcomitans. Within each family, all individuals who exhibited oral Actinobacillus actinomycetemcomitans yielded only isolates of the same serotype although the periodontal status of them are diffrent.