Although intraradicular bacteria are widely considered to be the primary etiological agents of periapical lesions, the immunobiological properties of the species in question are not adequately known. The aim of this study, therefore, was to investigate the pathobiological properties of the 10 most frequently isolated endodontopathic bacterial species. Using cellular components of the pathogens obtained by sonic extraction, we have investigated their ability to induce monocyte migration, interleukin 1 production, mitogenic responses of lymphocytes, and the polyclonal activation of B lymphocytes. It was found that all of the tested species enhanced the migration of monocytes and induced mitogenesis in B lymphocytes. The polyclonal activation of B cells and the induction of interleukin 1 by monocytes were found to be stronger in Gram-negative anaerobes. Furthermore, all of the tested species excluding Bacteroides oralis were poor T cell mitogens. These findings show that a wide range of pathobiological properties are attributable to the most frequently isolated endodontic pathogens. It thus seems plausible that a battery of complex immunological responses induced by such microbes lead to the formation of persistent periapical lesions.