The aim of the study was to see the total IgG and IgG subclass responses against Aa and Pg in the four early onset periodontitis (EOP) subforms or adult periodontitis (AP). 6 patients consisting of 3 patients from subform I (distinctive LJP pattern), 19 from subform II (post-juvenile periodontitis pattern), 16 from subform III (LJP pattern but rapidly progressing), 24 from age-matched AP (20-40 years of age) have been selected for the measurements of the total IgG and each IgG subclass against to Pg and the IgG subclass against Aa, respectively. The total IgG titers against to Pg of the subforms I & III had a significantly higher values than subforms II and IV (P<0.05). Among the IgG subclasses, only the lgG3 levels were significantly higher in the subform I than the subform IV(P<0.05). Wide ranges of the antibody titers were noted in all of the EOP subforms and the AP. Except for the subform I, which was typical of localized form, the IgG2 subclass levels to Pg gradually became higher in accordance with the subforms II, III and IV. Both of IgG2 and the IgG4 antibody levels of the EOP were significantly higher than those of AP, while other subclasses were not. All of the four IgG subclass levels to Pg were consistently found to be higher in the younger age group around 20. The levels found to be low around the thirties and then gradually became higher at the ages of late thirties. The IgG2 titer to Aa in the subform I was significantly higher than those of any other subforms. Combinations of IgG1+2+4 were the most frequently found to be elevated followed by the IgG4 only, the IgG2 only, the IgG2+4, the IgG2+3+4, and the IgG1 only, in the descending order.