The antibodies (C. pneumoniae-IgA, -IgG, and -IgM) in serum were measured by microimmunofluorescence while C. pneumoniae -DNA or C. pneumoniae -Ag of rat lung cancer was detected using polymerase chain reaction (PCR) or enzyme linked immunosorbent assay (ELISA), respectively. This evidence concerns the gene CD40LG and lung cancer.