Several oligopeptide segments of fimbrial subunit protein (fimbrilin) of Porphyromonas gingivalis strain 381 were synthesized and tested for immunobiological activities. Peptides F3(31-50; amino acid residue numbers 31 to 50, based on the amino acid sequence of the fimbrilin proposed by Dickinson et al., Infect. Immun., 170, 1658, 1988), F12(212-231) and F17(312-331) were found to be immunodominant epitopes of this fimbrial protein as revealed by ELISA. Furthermore, peptides F5(71-90) and F17(312-331) were demonstrated to agglutinate rabbit erythrocytes, and were mitogenic for BALB/c spleen cells but not thymocytes. These peptides enhanced the number of fimbria-specific antibody-secreting cells in BALB/c spleen cell cultures, and induced cytokines such as tumor necrosis factor-alpha and interleukin-6 production in human monocyte/macrophage cultures. The data demonstrate that these defined peptide segments are responsible for the immunostimulating portions within the fimbrial protein molecule.