Abstract. Juvenile periodontitis is associated with a high incidence of infection by Actinobacillus aclinomycetemcomilans (Aa). The data presented indicate that the ability of Aa to destroy human PMNs is altered during the course of infection. Leukotoxic strains of Aa are characteristically found in isolates obtained from younger patients (6–12 years of age) but not in older subjects (13–25 years old). This suggests that the leukotoxin may be more important during early as opposed to more advanced phases of juvenile periodontitis.