<qa>  How many subjects will be randomly allocated to 3 treatment groups?The aim of the present study is to evaluate the clinical and microbiological efficacy of moxifloxacin or amoxicillin plus metronidazole in one-stage scaling and root planing in treating generalized aggressive periodontitis. Forty five subjects will be randomly allocated to 3 treatment groups. Subgingival plaque samples will be analysed for cultivable bacteria. The primary outcome variable to determine the superiority of one treatment over the others would be differences between groups for means CAL changes at 6 months post-treatment. Secondary outcome variables include differences between therapies for the mean changes in the mean levels of PD and the proportion of BOP. 