Porphyromonas endodontalis is a black-pigmented anaerobic Gram-negative rod which is associated with endodontal infections and this microorganism possesses a potential for pathogenicity. The purpose of this study was to compare the membrane components of Porphyromonas endodontalis and Porphyromonas gingivalis and to study the immune reaction patterns of Porphyromonas endodontalis with patients with periapical lesion. Porphyromonas endodontalis (ATCC 35406), Porphyromonas gingivals serotypea (381), serotype b(W50), serotype c(A7A1-28) were cultured in anaerobic condition. Rabbit antisera were prepared by intravenous injection of formalized whole cells and human sera were obtained from patients and dental students. Indirect immunofluorescence method was used to study on the cross reaction between Porphyromonas endodontalis and Porphyromonas gingivalis serotype a, b, c antigen. Total membrane protein profiles of Porphyromonas endodontalis antigen were studied by sodium dodecyl sulfate polyacrylamide gel electrophoresis and the reactivity of antigenic components of Porphyromonas endodontalis against sera of patients and rabbit anti-Porphyromonas endodontalis antisera were assessed by Immunoblotting method. The following results were obtained : 1. Antigens of Porphyromonas endodontalis has multiple antigenic components, and both patients with periapical lesion and normal healthy individual showed immune response to this. 2. Patients group and healthy individual group showed a diversity of immune reaction pattern but they showed immune response against 43kd protein. 3. Patients with periapical lesion showed more diverse immune response than healthy individual and in some patients, much more bands appeared to lower molecular weight protein. 4. According to indirect immunofluorescence and Immunoblotting study, Porphyromonas endodontalis did not share common antigen with Porphyromonas gingivalis serotype a, b, c.