In the present study, serum anti-Bacteroides gingivalis (Bg) and anti-Actinobacillus actinomycetemcomitans (Aa) antibody titers in RPP, AP patients and healthy subjects were determined by ELISA. Discriminant analysis for differential diagnosis of RPP, AP and healthy subjects was performed using clinical measurements (PD, AL, PII, BI) and serum antibody levels as combined parameters. The results showed significantly different serum antibody levels among the three examined groups. The results also suggested that the combined use of clinical parameters and serum specific antibody titers may be a valuable measure in differential diagnosis of periodontal diseases.