Sequential changes visualized by scanning EM are described in hamster tracheal ring cultures exposed to 105 colony-forming units/ml of Clyde strain M-129 M. pneumoniae. Loss of cilia, elevation of cell borders and clumping of microvilli were induced within 6 h after incubation of the rings in media containing mycoplasmas and in sterile used media. The toxicity of Mycoplasma infection is apparently partially attributable to alterations in the growth medium or elaboration of a toxic substance by the organisms. Mycoplasmas were observed attaching to tracheal ring surfaces 8 h after infection. With increasing periods of incubation, cell death and alterations occurred in tracheal surface structure that might be confused with the organisms.