BACKGROUND: Chlamydia pneumoniae is a recently recognized species consisting of the strains commonly referred to as TWAR. These strains are associated with acute respiratory infections in humans, especially atypical pneumonia. So we tried to make a monoclonal antibody to Chlamydia pneumoniae. METHODS: C. pneumoniae were adapted to grow in HeLa-229 cells. The organisms were harvested and purified in a linear gradient of renograffin. BALB/c mice (female, 10weeks) were intravenously immunized with purified C. pneumoniae(TW-183).The spleen cells and SP 2/0 myeloma cells were fused with 40% polyethylene glycol (Mol.Wt.:1,450). Antibodies against C. pneumoniae were screened by an enzyme- linked immunosorbent assay (ELISA). The proteins of purified chlamydial elementary bodies were separated by discontinuous sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and Western blots were performed with these monoclonal antibodies. RESULTS: Two monoclonal antibodies (HYMD10, HYMG12) reacted specifically with C. pneumoniae, as measured by an ELISA and indirect immunofluorecent stain. One of the monoclonal antibody (HYMD10) reacted with 75- and 39-KDa proteins in Western blot. The other monoclonal antibody (HYMG12) reacted with 98- and 39-KDa proteins of C. pneumoniae. CONCLUSIONS: These species-specific monoclonal antibodies (HYMD10, HYMG12) to C. pneumoniae could be used for diagnosis of C. pneumoniae infections.