Mycoplasma pneumoniae induces pneumonia-like symptoms in hamsters and causes ciliostasis and cytonecrosis in hamster tracheal explants. 2,4-Dimethylphenol and, to a lesser extent, its 2,3-, 2,5-, and 2,6-dimethylphenol isomers protected tracheal explants from these changes after exposure to virulent M. pneumoniae strain PI 1428. The effect was concentration, time, and isomer dependent. At concentrations of 10(-9) M or greater, 2,4-dimethylphenol completely prevented the morphological (loss of ciliated cells) and biochemical (decreased dehydrogenase activity) changes normally observed after exposure to M. pneumoniae. Apparently, 2,4-dimethylphenol interfered with an early event in the infection process. Complete protection required that it be present during the first 2 h of exposure of the explants to the infecting mycoplasmas. These xylenols may prove to be useful tools for helping to define the mechanisms of pathogenesis in certain respiratory infections.