Since steroids are powerful anti-inflammatory agents and increase susceptibility to a variety of infections, including Chlamydia (Chlamydophila) pneumoniae respiratory tract infections, the effect of the steroid receptor antagonist RU486 (mifepristone) on C. pneumoniae growth in epithelial HEp-2 cells was examined. Treatment of HEp-2 cells with RU486 significantly inhibited the growth of C. pneumoniae in a dose-dependent manner. Electron microscopic studies also revealed that the treatment of infected cells with RU486 resulted in a marked destruction of infecting organisms. The addition of the host cell protein synthesis inhibitor cycloheximide to the infected cells did not alter the inhibition of C. pneumoniae growth by RU486. Pretreatment of C. pneumoniae organisms with RU486 before addition to culture also did not result in any modulation of bacterial growth in the cells. However, the binding of RU486 to C. pneumoniae organisms in cells at 24 h after infection was demonstrated by immune electron microscopy with anti-RU486 antibody. Incubation of cells with anti-RU486 antibody completely diminished the inhibition of C. pneumoniae growth by RU486. These results indicate that RU486 may directly bind to the bacteria within cells and cause the destruction of C. pneumoniae. This novel mode of regulation of C. pneumoniae growth in cells by RU486 might provide a new approach to understanding complicated aspects of C. pneumoniae infection.