P>Methanobrevibacter oralis is an archaeal species frequently isolated from sites of severe periodontitis. However, its pathogenic roles remain unclear. Here, we aimed to isolate group II chaperonin from M. oralis and examine its antigenicity. The genes encoding two chaperonin subunits (Cpn-1 and Cpn-2) were cloned from M. oralis using polymerase chain reaction and genome walking procedures. Recombinant proteins Cpn-1 and Cpn-2 were generated, and the reactivities of sera from patients with periodontitis were examined by Western immunoblotting. The open reading frames of Cpn-1 and Cpn-2 genes consisted of 1641 and 1614 base pairs, respectively. Putative ATP-binding domains conserved among the chaperonin family were observed in both genes. The deduced amino acid sequences of the two genes showed 28.8-40.0% identity to each of the subunits of human CCT (CCT1-8). Thirty and 29 of 36 patients' sera reacted with the recombinant Cpn-1 and recombinant Cpn-2, respectively. Western immunoblotting using antiserum against human CCT subunits indicated that anti-CCT3 and anti-CCT8 antibodies recognized recombinant Cpn-1. In addition, anti-CCT1, CCT3, CCT6, and CCT8 antibodies recognized an antigen of approximately 60 kDa in M. oralis. The results suggested that the chaperonin subunits of M. oralis were antigenic molecules that were recognized by periodontitis patients and that may cross-react with human chaperonin CCT.