Twelve patients of localized juvenile periodontitis were evaluated to detection of serum IgG and IgM antibodies to Actinobacillus actinomycetemcomitans Y4 strain. Sera were isolated from those patients. Antibody titer of patients sera to Aa stran Y4 strain. Sera were isolated from those patients. Antibody titer of patients sera to Aa stran Y4 were determined by modified enzyme-linked immunosorbent assey (ELISA) using sonicated and formalin-fixed whole Aa y4 strain for detection of serum IgG and IgM antibody titers. To compare with health control and L.J.P., we used 12 healthy dental student who did not exhibited any gingivits. Results were determined by using ELISA reader at 400mm absorbance value. Data analysis were performed with comparison of the regression functions relating absorbance to dilution and Dunnett t-test. Significant high antibody titer to As Y4 in L.J.P. sera were shown in this examination(281. 4 Eu-G to 162.80 Eu-G, 106.0 Eu-M to 40.0 Eu-M for sonicated As Y4 antigen and 653.960. to 138.117 Eu-M for intact As Y4) and this data were also statistically significant (P<0.05). This work was supported in part from Seoul national University Hospital Grant and Korean Science Foundation.