Objective: The use of antibiotics as an adjunctive therapy in the management of periodontal diseases has always been of interest for dental practitioners. Although studies have demonstrated the importance of different antibiotics as adjunct in the treatment of periodontitis, few studies have reported comparative results of antibiotics in localized aggressive periodontitis ( LAP). The aim of the present study was to evaluate the comparative effects of systemic tetracycline derivative doxycycline versus a combination of metronidazole and amoxicillin in the elimination/suppression of Actinobacillus actinomycetemcomitans and their clinical effects during 90 days of follow-up in patients with LAP. Method and Materials: Thirty patients with untreated LAP participated in the study. Patients were randomly divided into 2 groups and given doxycycline or metronidazole plus amoxicillin, and periodontal clinical parameters were achieved at baseline and 10, 30, 60, and 90 days after microbiologic sampling. Patients were also given mechanical debridement after measurement at baseline. Results: Both groups of patients demonstrated improvement in clinical parameters, and microbiologic samples did not show any A actinomycetemcomitans at days 30, 60, and 90. Conclusion: Systemic use of doxycycline or metronidazole plus amoxicillin as an adjunct to conventional therapy may be used for the treatment of patients with LAP.